I'm a newbie reloader and I'm having trouble when I resize my cases. Everything seems to be fine but I'm not getting proper neck tension. The caliber is 257 roberts. I'm using a basic Lee C-press and Lee Pace setter dies. Everything seems to be fine until I try to seat my bullets and it drops right through.

Any suggestions on what I'm doing wrong? My 308 with the same setup has no problems. Could it be the die?

Could be the die, the expander or the brass.

The die might not be compressing the neck enough, the expander might be to large in dia and the case neck might be to thin. I've never used the pace setter dies but if you can remove the expander and see if it makes it better. Then we can go from there.

It has to be the die.
I'm not familiar with the Lee dies but with the exception of some bushing dies all threaded dies first size the case and in your case most importantly the neck and then on withdrawing the case there is an expander plug on the de-capping rod that stretches the neck back out a bit to the proper size.
My first guess would be your expander plug is way to large and is expanding the neck too much. Second guess is the die neck isn't sizing the case enough as in a bushing die without a bushing, or just a screwed up die cut to the wrong dimensions.
Last guess is the die is set out way to far and not sizing the case at all.
Did you follow the instruction on die set up that came with the die?
Let us know what ya come up with.

Thanks for the responses so far. I think there may be something wrong with the die. It's very difficult to get the neck in an out of the die. I can only compare it to the 308 die I have that goes in an out very smoothly.

Use your caliper and measure the OD of the case neck before and after sizing. Measure your expander ball and a bullet. The expander should be 2 to 3 thou smaller.

Mic the neck on an unsized case.
Pull out the expander/ decapping rod.
Size case and re-mic neck.
Will it hold a bullet now?

You could have a .25 caliber die neck and a .27 caliber expander plug installed by mistake, stranger things have happened.

If when the decapping rod is removed and you size the case without expanding the neck and if a bullet still slides through then, you have a "FUBAR" die. Send it back.

I couldn't get the Lee die to come apart to take a look. I bought a Redding Die set since it was the only non Lee brand available in 257 Roberts. I tried it today and have no problems with neck tension. I must have got a defective one from Lee. It's not worth sending back as the set was only $25. My 308 Lee die set seems to be working fine.

I'd send it back anyway but that's just me. Glad ya got your problem resolved but I still can't help but wonder what the issue was.
Have a good one.