I found on mine that after I adjusted the gap and tightened the thumbscrews, I had to tighten the nuts with a small wrench to keep it set where I wanted it.
My guess is that you did not cut the tube to the proper length. It should be just long enough to reach the bottom on an empty keg. I have these in all of my kegs and none of them needed added weight.
I think you should have pitched the whole packet. Lagers need about double the yeast you would use for ales and it never hurts to use yeast nutrient. There is a newer yeast called NovaLager that allows you to brew a bit warmer and it uses ale yeast pitch rates. I have been having good luck...