Without seeing the guitar it is difficult to assess the problem. Most tuning machines won't slip. Slippage is usually between the string and the machine. It can also occur at the bridge. Make sure that the string has enough turns around the post and has at least one overlap so that it will bite on itself.

At the bridge end make sure that there is a slot in the bridge so that the ball can catch under the bridge and not pull the pin out.

If none of that works it may be cheaper to salvage the machines from a thrift store guitar than to buy new ones.

Also some authorities suggest that you never lubricate the tuning machines, but I really don't recommend that.