If your machine heads are of the cog and worm type, they don't slip. They can't. Not unless there are teeth missing from the cog or there is severe damage to the worm. Both of these should be easily spotted.

What can slip is the string on the tuner barrel. If the barrel is scalloped, i.e. thinner where the string goes through and gets wider above and below that, I fill all the scalloped area below the hole with neat winding when the string is tuned to pitch (as near as possible). This acts like a string clamp as the strings at the widest part of the barrel are forced upwards by the string tension and clamp the string against the top of the hole. This will only work if all the windings are in contact with the barrel. Quite often the string bends out before it goes through the hole and this needs to be pushed flat or it will slip. Each string needs to be pulled several tim es to get both ends to bed in fully. After all that they should not slip at all.

Don't forget to lubricate your tuners and to check for loose screws.