First of all make sure that the slit is clean and if they are still stiff maybe some candle wax could help. Just run a candle along the edge of the slide, no melting and pouring. Cleaning the slit should however be the first stop.
I use paste wax on the entire slide but making sure the slot is clean, and lubricating the edge of the slide, as above, is the first stip.
If you wax the surface of the slide, make sure it's thoroughly buffed before inserting it into the holder.
Even simpler and less costly is "Nose Grease". It works beautifully and unlike paste waxes does not leave a layer of wax to gather dust and grit which will sooner or later cause problems.