Do you have the 3 die or 4 die set from Lee? Either way, back your seating die up so that the crimp portion of the die is out of the way. Run the seating plunger in however far you need to in order to seat the bullets. If it's a 4 die set, crimp with the Factory Crimp Die that came with it. If it's the 3 die set, back the seating plunger way out so it doesn't come into contact with the bullet and crimp with the seating die as a separate operation.
Also, if it is actually the crimping portion of the die that is pushing the bullet down, I wonder if the WFN design you're using isn't a little too wide, especially if it's still seating bullets with the seating plunger removed from the die completely. It may not give a problem with the 240s because they're shorter than the 280 grain bullets (speculation on my part). That said, if you were able to seat and crimp the 240 grain bullets properly and are having problems with the 280 grain bullets, I'd wonder at the bullets before I'd blame the die. Do you have the same problem with 280 grain bullets of a different shape?