Assuming you are using the recommended method of precoating slides with agarose: Don’t go back to using frosted glass slides, or expensive commercial coated slides. Gels falling off is an occupational hazard, that happens to everyone at some time. Usually whatever you do to cure the problem has no effect, and then one day the problem goes away. This is frustrating; it is impossible to give a sure solution. Some claim that the slides should be dried on the open bench, rather than at high temperature. Others find that if the laboratory atmosphere is humid, the gels do not stick as well as in a dry atmosphere. Another observation is that leaving the slides in lysis solution for more than a few hours makes the gels fall off – but only if the cell suspension contains a trace of medium with serum.
You could consider using plastic film (Gel Bond - available from Lonza) instead of glass slides. The film costs more than slides, but gels never fall off – providing you use the hydrophilic and not the hydrophobic surface.
"FAQ by Andrew Collins, Gunnar Brunborg and Jonas Nygren, 2006, NewGeneris FP7-project"