I think I know what the problem is as I used to be in the exac same boat you are in now. I am willing to bet you are, in fact, playing too tightly at the low limits and not seeing enough showdowns (read: not playing optimally postflop). I am not sure of your aggression, although I imagine it is fairly good, as you said you can beat the $10+1 and $20+1s. This might be where the problem stems from, as the SNG's teach you to play excessively tight the first few rounds and to avoid unneccessary risk.

This is one quality that doesn't translate over well to limit hold'em. In low limit ring games (and I assume you are referring to full ring), you need to take advantage of more spots where the pot is laying good odds with all those bad players, both pre and postflop. You should go ahead and purchase PT, as it will tell you exactly where you stand as a player and what needs to be changed.