Single speed conversion chain keeps derailing rear cog under load? The chain on my SS mt bike keeps coming off the rear cog under load (like when I hit a steep hill) it will hold right after it's tightened but after a few hills it starts coming off non-stop. Talked to the LBS they so far are just telling me to tighten the tensioner, I was thinking i might need a bigger rear cog for more "chain wrap" any advice.
Some possible culprits:


cog and the chainring are not in line.
rear hub is not tight enough and twists under load (see 1.)
chain is stretched or cogs worn out - so the chain doesn't 'seat' well in teeth
what the shop says - chain not tight enough