My storage heater fuse has recently started to burn out. I put covers on them around 2 months ago and there is no problem with them except one. It has progressively got worse to the point where it is burning out without even beginning to heat up. I assume this may be to do with the covers, but then surely all the other heaters would burn out aswel.
They have vent holes to let the heat out - if you cover the air holes then it will get too warm inside. Your fuse - probably a thermal link fuse - will burn out.
As for the ones not popping the fuse who can say - they too should not be covered there's a fire risk - that's why electrical appliances have fuses.