Gatomon was unable to digivolve to Angewomon because of the control spires that Ken built while he was the digimon emperor. The spires did two things, provide power to the dark rings that were used to enslave digimon and prevent natural digivolution (baby->in-training->rookie->champion->ultimate->mega). This is also the reason that none of the other digimon were able to digivolve to their champion level during that time. This meant that they had to find the digi-eggs to armor digivolve (armor, DNA and dark are digivoluions that branch of the normal path). In terms of Gatomon losing her tail ring it lowered her power from champion level to the level of a rookie and enabled her to armor digivolve and is later found and given back to her by Gennai. As for the tags and crests from the first season, they were focus points (the crest power was inside the digidestined that had it) that made it easier to digivolve to the ultimate level which is why the digimon were still able to digivolve after Apocalymon destroyed them at the end of season 1.