This is a new obstacle for me as a learning jungler that has stopped my progress. When I'm behind I can't gank very well or be much help in an team fights. I'm usually stuck on the decision to either power farm or gank for the lanes especially the ones that are behind. I also get a bunch of harassment from lanes for not ganking which flusters me even more. I don't know what to do at this point so please any advice would be great.
If you are behind, never waste time on a gank, because if you are behind your ganks are much more likely to fail (or worse, lead to you/your laner getting killed instead).
Hecarim is a very fast clearing jungler, so if you are behind, spend 5 or so minutes hard farming the crap out of your jungle to catch up in gold items and exp. If you're still behind after that 5 minute stint, farm some more, only stopping if your team needs to push a tower or take an objective.
Later on in the game you can also split push exceptionally well because of your wave clear and multiple escape tools, so even when behind, forcing down a wave is a good wave to get objectives and free CS.
Play him top lane, because currently Hecarim is sub-par in the jungle
Like Junglerboy16 said, no use waisting time on a gank when your behind. If your so behind that your in no position to carry your best bet is to get your teamates fed so they can carry the game
