Might try negative chins for 12 or so reps. Stand on a bench, box, etc about chin level and just lower the body down slowly to a full stretch bottom position. Add weight as you get stronger. After a couple of weeks try to do your regular chins. Reps will increase.
Get a dip/chin belt, add about 20-30 lbs, and only chin to a little below the half way point of a regular chin. Try for 8-10 reps. Second part of this chin training is to start from about the half way point and complete a chin to the top position, with added weight on the belt. Some of the stronger men I've seen have used over 300lbs for quarter chins from the bottom position. This is called partial rep training, usually used in a power rack. Your chin can gain a alarming increase in strength training this way. Your regular bwt chin should shoot up like a rocket after a few week. In either the negatives or partial chins do not preform a regular chin in your workout until after a few weeks. Just fron doing the negatives and/or partials your strength and muscle mass should increase. Good Luck.