Vovin, welcome to wax injecting. Don't knock the cheap little hand
injector - it will do a good job for small shops, but you need to
learn good techniques. The wax sticking to the rubber means your
wax temperature is way too high. Check the chart of the wax you use
and, if necessary buy a special thermometer to make sure you stay in
the range recommended by the manufacturer. The raised letters
vanishing sound like you are compressing the mold too tightly,
squeezing off the expansion as the wax is forced in. Relax your
grip - or the mold holder clamp's. That should solve your problem.
If it doesn't, slicing another small vent from that area could do
the trick. But look to your pressure first. It all takes a bit of
trial and error for a beginner. Suggest you get a small notebook
and record all settings for each mold and note where it works
perfectly. Relax and experiment, don't fight that machine, enjoy it.