You cannot just wax over an old floor. That floor must be properly cleaned and stripped first. Dirt, grease, dust, grime and other residues that litter your VCT tile will serve as a barrier between the new coat of wax and the surface, meaning that it won't stick properly. Clean the floors thoroughly using a scraper to scrape up things like hardened bubble gum or large sections of grime and use powered scrub brushes to remove any residues. Do a final sweep to collect any remaining traces of dust or dirt.
The old coat of wax is the second barrier between the new coat and the VCT tile. The tile's present wax coating will not allow the new wax coating to stick properly, so you'll have to remove it through a process known as "stripping." Use a special chemical known as a floor stripper that can be found at virtually any hardware store to remove the old coat of wax according to the manufacturer's specific instructions. You must typically dilute the floor stripper with water and apply it with a mop, then let it sit for several minutes while it eats away at the old coat of wax before mopping everything up.
Water may be another reason your new wax coat isn't sticking. There may be excess moisture over the VCT tile's surface after you've cleaned it and stripped it, which will inevitably get between any new wax and the flooring, itself. Vacuum up any traces of water with a wet/dry vacuum and go around with a dry mop until the floor is completely dry and spotless. It is imperative that the floor is totally free of dirt or other residues, as well as the old wax coating, before attempting to apply the new coat.
Apply the floor wax properly and evenly so it sticks. The temperature should be over 54 degrees Fahrenheit for most types of floor wax or it may not adhere to the flooring. Always follow the manufacturer's instructions pertaining to your specific type of VCT wax or finish and always wait for a coat to completely dry before applying a new coat.