I'm building my first Loki 76 (the L-930), and I am having trouble inserting the smoke grain. I am trying to follow the instructions. The O-ring on the smoke grain prevents it from going all the way in. No matter how hard I push, the grain pops back out. I have a light coat of grease on everything. I have the O-ring at the forward end of the bulkhead like it shows in the instructions. What am I doing wrong? Thanks.