The slide may be sticking for any one of several reasons. If you overtighten the screws (one or both) holding the mouthpiece on, the slide will stick. If you've dismantled it and bent any of the sections, it will stick. Gunk, aka dried-on saliva complete with its array of nasties, will make it stick, as will old goo such as Vaseline. Don't dismantle it just yet. Try this first: Fill a shallow container with HOT water. Hold the harmonica, mouthpiece down, in the water to just cover it up to the slide mechanism. Work the slide rapidly back and forth under the water for a little while. Remove from the water THE SAME WAY UP, then shake/tap out excess water. If this doesn't work, it will mean dismantling and a thorough clean. Get most of the dirt off with soapy water, poking away stubborn deposits gently with a matchstick or similar, then rinse. The combination of harmonica and toothbrush is an accident waiting to happen. A drop of isopropyl alcohol (the stuff that comes in little bottles with record cleaners) will shift greasy remnants. My underwater cleaning method, used after every session, has meant that I never have to dismantle the slide mechanism. I would counsel against using any lubricant at all. They all do the same thing: attract dirt. Good luck!