An article by George Ledyard Sensei that addresses this issue was posted here not too long ago.
A couple things that come to mind for me:
- Every teacher I have ever worked with has said, if the technique you are supposed to be practicing doesn't go, do something else -- don't stop.
- Any technique is stoppable if uke knows what nage is going to do. So, you may occassionally have to do something different to keep uke on his/her toes.
- Sometimes, an atemi can help point out to uke that in trying to stop your technique, they are creating an opening they might not want to create.