I'm sorry to say, but you're hit by a common problem which has been repeatedly reported here. I have some savegames available which can help you to overcome this problem.
Level 15 start file:
This will help you to get going, but after a while you'll encounter more problems. Then the following files will hopefully help you:
In case you're interested what others here have experienced:
Anyway, after you finished level 15 the next one(s) again will work correctly. As far as we all here found out, these problems only seem to apply to level 15.