There are a number of reasons for a book running out of steam, but the most common one is that you don't have enough plot. Some of this is covered on the Planning page, but basically your plot needs to have enough elements in it to sustain a whole book.
If your book is running out of steam, ask yourself the following questions:
If the hero is trying to catch a spy, then he will have various suspects in mind and he will perhaps follow them from time to time. This could lead to him seeing the suspect with the heroine, which will add more complications to your book. It will make him wonder if the heroine is somehow involved, which will make him treat her coolly and with suspicion when they meet on social occasions. This can then make her wary of him, and lead to a later scene where they overcome their coolness and distrust, perhaps by physical contact - in which case, when it's over, the coolness will remain - or by discovering they can trust each other, eg someone has a gun on the hero and the heroine comes upon them unexpectedly and hits the gunman over the head.
If he is trying to find out who framed him, he will have suspicions and will perhaps be rude to someone because of it. The heroine could wonder why he is so rude and even argue with him about it, telling him that just because he is the new lord of the manor doesn't mean that he can treat other people with contempt. This will probably make him admire her decent character, but he will not be able to explain matter to her without revealing that he was framed. He can do this later in the book - it will be a powerful scene that the reader will look forward to - but earlier on he can decide not to tell her because he doesn't trust her, and assumes she will believe the lies told about him, just like anyone else.
If the heroine is trying to catch a husband, then she will need plenty of reasons for falling foul of the hero. Perhaps he despises her for what she is doing and she is too proud to tell him she has three needy little siblings at home. Perhaps he then follows her one day as she rides out of town and sees the poor house she goes to, and the ragged children who run out to hug her. He can then change his opinion of her, but he cannot let her know that he has seen her, because he doesn't want to destroy her pride. Perhaps he then helps her, introducing her to some eligible men, whilst gradually coming to realise he wants her for himself.
In each of these examples you will see how one thing leads to another. If you have an overarching plot, then think around it, asking yourself:
How will this situation affect my characters' behaviour? What will they do? If they are tracking down spies, they will obviously have to follow people etc.
How will it affect the way they relate to other people? Someone tracking down a spy will be suspicious; someone trying to catch a husband will set out to captivate.
How will it affect the way the hero and heroine relate? This should change throughout the book as new dimensions to the plot, and their characters, are revealed, as in the example above when the hero changes his opinion of the heroine, realising she has a young family to support.
Come up with as many likely situations as you can - situations that tie in with your main theme - and jot them down. Then see how you can have the hero and heroine together in the situation. When you have done this, you will be well on the way towards solving the problem of running out of steam. Ask yourself how your hero and heroine will feel in this situation. How will it affect their actions? How will it affect the things they say to each other? How will it affect the way they feel about each other? Will they part wanting to see more of each other, or wanting to never see each other again? Or will the hero want to see more of the heroine, but will she want to never see him again?
There are other things you need in a book, and these will all take up space, making sure you don't run out of things to write about. if you are still stuck, ask yourself this:
If you do, make sure something happens, to give the scene extra depth.
Here are some ideas.
Something one of them sees reminds them of their past and they comment on it. This can lead to them learning more about each other. For example, on a picnic, one of them can say 'I haven't been on a picnic since . . . '
Their counterpart can prompt them, and they can reveal something from their past. Try and make the revelation relevant to your story. If your story is about spies, perhaps the hero hasn't been on a picnic since he learnt that his brother had been killed in the way on a previous picnic. This would give him a personal reason for being determined to catch spies - perhaps spies had given away information which led to their brother's death. Persnal reasons for actions will always add depth to a character.
If your book is about the hero trying to find out who framed him, then the scene would being the same, but the hero would say ' . . . since the ring was stolen.' Or perhaps, 'Since my father accused me of stealing his ring.'
If the heroine is trying to catch a husband, she could say, 'I haven't been on a picnic since my father died. I used to go all the time. I went to so many parties . . .' She smiled at the memory. 'But once he died, everything changed.'
If used in this way, the scene deepens the story, because it tells the reader more about the characters, their backgrounds and their motivations. It also gives you more to write about!
For more detailed suggestions, see the Advanced Planning page.