You mention load 250 sucsessfully, Is this the first time you are loading on you own?
I mean why now a used set of dies when you had loaded before with new ones?
If you are loading on a single stage press the only reaason I can think that your cases are colapsing is not enough bell.
There is a SLIGHT possability the bullet is getting cocked a little before it enters the case. Are you guiding the bullet into the die as you operate the handle?
If these don't help, more details will be necassary to help