my charmeleon won't rise in xp after he's reached Ivl 20, why is this?

[evyjelle]

my charmeleon won't rise in xp 
after he's reached Ivl 20, why is this?

[pvstelsouls]

You’ve reached the level cap, which stops your Pokemon from levelling up after reaching a certain level. The level cap rises after defeating a gym leader (most of the time), and your current level cap is displayed on your trainer card. 

[Shadow Angel]

Note that Rare Candies will still work to rise above the cap, but any Pokemon above the cap will no longer obey you (like traded Pokemon in the official games, but applies to all Pokemon in Rejuvenation).

You can take advantage of this to get a level-up move or evolved Pokemon before the level cap rises to the needed level, as you can use Reverse Candies to de-level your Pokemon back below the level cap, and then your Pokemon will obey you while keeping the move/evolution obtained at the higher level. I did this to evolve my starter for the very difficult ghost gym battle, where the level cap is 35 but my starter evolved at 36.