What's the temperature of the water? What are you feeding them? Do you have a calcium source for them?
I have a 2.5 gallon tank for my ramshorn snails, it's not filtered but it does have a heater. I keep the tank around 75 degrees, but before I got the heater the tank would get down to around 67 degrees at night, I have found that when the water was at 67 they moved very slowly. I have live anacharis in that tank with them, and some guppy grass, to help keep the water clean. I clean the gravel and do a 50% water change around every 2 weeks.
For food around once a weeks I'll drop some algae wafers in the tank, I'll also sometimes put in decaying leaves from the plants in my 5 gal tank, or a couple of blood worms, or Omega One pellets. Overall they do pretty good, and the3 adults are probably around 8 months old by now, I raised them from babies.