My tenant has the same problem, but I believe it's because she keeps the heating off most of the time in the winter, due to the expense and also leaves wet towels to dry on radiators, when they are on. I think she is to blame, but I can't make her keep the heating on.
If you are in the UK, your landlord is not supposed to be holding your deposit, it has to go into a special deposit scheme. This is to protect tenants from unscrupulous Landlords, who just want to pocket their tenant's deposit without good reason.