pull-ups use a lot of muscles: lats, traps, rhomboids, biceps, and forearms. if you're convinced your biceps and forearms are strong enough, then you need to train your back. my guess is that your lats are weak.
In general, when strength training, you'll want these two exercises as part of your regiment:
* Squat - builds awesome overall body strength and should be the core of every real strength training routine. also reasonably good for pull-ups as it uses a lot of back muscles to hold the weight.
* Deadlift - again builds overall body strength, but this uses even more back muscles to keep your spine rigid against a heavy load.
As for pull-ups specifically, you can try:
*
Pull downs: very similar motion to pull-ups, except you can start with a lower weight and work your way up.
*
assisted pull-ups: lets you do pull-ups, but provides some counter weight if you can't do your body weight. if you gradually decrease the amount of counterweight, you'll be doing pull-ups in no time.
* You can also change your grip on pull-ups to see if one of the alternatives is easier so again you can work your way up. You can try pull-ups with your palms facing you (aka chin-ups) or even put your arms parallel to each other. There is no "bad" pull-up and if you can get strong at any one of these, the others will develop too.