Coarse substrate and snails going inactive?

[lla]

Heya! So, my snails have started being less active, at first I thought it was because of the colder weather but I've come to realize it's because of the substrate. Right now they are on dirt, the same kind you would use to plant flowers (I know I know, not the best substrate but it's what I have at the moment). About two weeks ago I decided to plant some grass in the substrate and they sprouted beautifully. I've now realized that even though I mist the tank often the substrate has become coarse. I've tried to remedy this by adding plenty of water, almost soaking the substrate (I made sure it wasn't soaked enough to cause any danger to my two snails). I was wondering if anyone has any idea what else I could do to deal with this issue? I currently mist the tank once or twice a day. The tank is also plastic and close to the heating system in my room so as to keep the temperature higher due to how cold it's getting. I've made sure the tank itself doesn't get too warm though. My two snails have also stopped eating for the last couple of days, which has me quite worried. One of the two, the smaller one, has also been in the top corner of the tank for several days. Help? 

Extra note: I am almost certain they are both White-lipped Banded Snails (Cepala Hortensis). 

[pasi9]

Sometimes the substrate can be too wet. A good rule of thumb is to squeeze a handful of soil, and if more than a couple of drops come out then it's too damp. If nothing comes out, it may be too dry.

They probably don't mind the substrate being coarse, but does it have fertilisers in? Because if so, that can be bad for them.

[lla]

I'm pretty sure it doesn't, but the substrate just felt dry and coars, a bit like (forgive the comparison) if you put blood on a carpet and let it dry, then it's kind of just solid? I'm not sure how to explain it. I think tomorrow I'll go check again on the ingredients that were in the substrate, and possibly add a new layer to the substrate since it's not very deep.