It sounds like you are trying to use the wrong vowels for the vocal mode you're attempting to sing in. Another possible reason that your falsetto is hard to sing in is that if you only warm up your falsetto, you will only be able to sing breathy. If you only warm up your chest voice, since it is created with a different muscle group, you will not have access to your high range at all and will either crack or no sound will come out when you try to use it. If you only warm up with twang but not your chest voice or your falsetto, your voice will sound very thin and disconnected. You need to warm up all three muscle groups and drink plenty of room temperature water to get the most out of the range that lies above your first passaggio. You also need to practice transitioning from one vowel to another. The Zee-ay-ah-ee-oh-ay-ay-lay-mine exercise from Jim Gillette's Vocal Power will help with that.

One last thing: You only really mentioned falsetto in your post, so here's a question that might better determine what is causing the collapsed falsetto issue: how strong does your chest voice below the break? The voice is like a tree, with the chest voice its foundation (roots). If the middle voice is the trunk, then the falsetto would be the branches. So you want to get that chest voice strong in the low and middle range, and make sure to shed some of the vocal weight as you go up in pitch. If you shed too much weight, your voice will be a breathy falsetto. If you don't shed enough, it won't be able to ascend because the chest voice musculature will be overpowering the other two muscle groups.

Hope this helps.