The pop should not be there. It should be a smooth release on the upstroke. I think you need to do a few things. Firstly, take the expander die out and clean the plug and the die to be absolutely sure there isnt a piece of crud in there. Then have a look at your Powder measure and spring mechanism. I am pretty sure the problem lies there. If the assembly is not straight vertical it can hang up as it can also do if the tension on the T or L bar isnt right. It may be as simple as the springbushing being in backwards but it should not hang up. Those pops can knock the powder around pretty good and give u false throws and in some cases, the disc is not fully returning resulting in no powder being thrown at all. Do some dry runs and you will determine where its hanging up. I was plagued by squibs as a result of this very thing and I did manage to solve but it took some time to diagnose properly.
Good luck
dB