What tea are you using? I use English Breakfast tea and have always had copious amounts of carbonation (less so if I use green or white tea). Some second ferments that help with carbonation are anything lime/lemon, ginger or the tropical juice mixes that have mango/kiwi etc. Do you leave your filled bottles out for a couple days after adding the juice?
The primary mistakes I have seen are people trying to reduce sugar too much or to use sugar substitutes (like stevia or coconut sugar). The scoby needs sugar to feed off and to keep everything in balance. When things do get off balance you are likely to get mold, which could be harmful. If you see mold, toss the whole lot and start again with a new scoby. Luckily, mold is pretty easy to detect. If something is furry or powdery, there is a good likelyhood it is mold.
Slimy brown globs are normal yeast and nothing to worry about.