[cantgainmuscl]

I have tiny inner forearms and can't gain any muscle on them.

It seems that moderate to heavy weight does nothing for strength or muscle.

(hammer curls) When I do very light weight very slow with full range of motion and squeezing to failure It seems to work but I can't be certain.
If I even miss a day it seems like to tiny amount of mass I gained goes away super fast(for most of my upper body for that matter)

Also my biceps are very hard to train as well. Any advice on that as well?

My upper body is small boned. It is very hard to push my limits or know how far to push them with out injuring myself.

[PaulBlack]

I have tiny inner forearms and can't gain any muscle on them.

You are not alone. Quite a few smaller boned and longer limbed guys have this situation.

It seems that moderate to heavy weight does nothing for strength or muscle.

Well, it does, but not as much in a fashion of say small iso exercises or just getting that bodypart to rise to the occasion. In general, your body will grow in accordance with other body parts. Unless a person is chemically assisted, most average guys/girls will not be able to over size one single smaller body part, and not have other larger parts effected. It's kinda like not being able to spot reduce fat either.

(hammer curls) When I do very light weight very slow with full range of motion and squeezing to failure It seems to work but I can't be certain.
If I even miss a day it seems like to tiny amount of mass I gained goes away super fast(for most of my upper body for that matter)

Well, forearms are a super pumpable quick to swell muscle group. You can pump them up a bit, but by the next day the swole is gone again.

Actually, it seems better and from my experience it was better to focus on the bigger exercises and gain overall bodyweight, then my arms, forearms etc. kinda followed suit. If you add 10 or 15 pounds to your entire frame, your whole body will be most likely larger all over. Some people don't think #10-#20 pounds is much, but try eating #10 of meat. It is a lot of tissue.
Biggest thing is not to stress over the smaller muscles and perhaps try working on your larger muscle groups like legs, back, shoulders n' chest. Then the arms etc. will grow to keep up with your growing BW and you can use the iso exercises to better advantage at the end of the workouts...!
Personally, I have gotten more for my biceps at times by doing full range weighted chins (or pull ups with supinated grip) The kicker is, that I was training my back/upper lats and the arms got bigger as a byproduct.
Hang in there and don't give up...!