- Validating your steam install (reinstalling)
- Trying the LW campaign instead (I assume you are talking about the RAF campaign?) and see if the problem still happens (the LW campaign is better anyway - less stupid plot, fewer botched mission designs)
If a reinstall doesn't solve your problem there is a way to play the missions individually though personally I think this is a lot of work, for what is a disappointing campaign in the end, but that is up to you:
Go to the campaign folder (usually C:\Program Files (x86)\Steam\SteamApps\common\il-2 sturmovik cliffs of dover\parts\bob\mission\campaign). Here you will find a folder called campaign_GB, which is the RAF campaign. COPY it.
Go now to the single mission folder (usually My Documents>1C>IL2sturmovikCliffsofDover>missions>si ngle) and PASTE the campaign_GB folder in there
Now, when you go to the Single Missions Menu, you should see the campaign_GB folder there, and you will be able to fly all the missions there individually.
Of course, if you want even more fun, you should rather download and install some user created campaigns instead.
I recommend going to www.airwarfare.com and downloading campaigns from there. Or you can try my own single mission packs such as