I bought a used 5 Gallon Low Profile Keg from homebrewing.org and wanted to change the poppets that came with the keg.They had three prongs and were lodged in the fittings. I had to get a phillips screwdriver and hammer to get them out. I went to the LHBS with the fittings and bought 2 new 'generic' poppets, they are approx. 3/4" in length. I also replaced the original o-rings with the ones that homebrewing.org sent. Now I can't get the fittings to screw onto the keg. After removing the o-rings, the fittings will go on the keg but there is a gap (where the poppet should be flush with the top of the fitting). I'm at wits end, what can be wrong?