Hello all...
Well, today i was training with my friend, he's a mage, and the intention was, while my skills grow (cause i'm warrior) he trained magic level, with healing spells...

I was attacking my friend, and 3 monsters was attacking me, my melee skills was growing normally...but my defending skills stopped to UP, why?
Someone can answer me?
Please, help me with this, and tell me Tricks about training...

Thanks all :)

Because skills only go up for 30 seconds with monsters.

I think the problem may be your mixing of monster and player training. As Snirf said monsters will only increase your skills for 30 seconds, after 30 seconds you need to switch target to another mosnter, and after another 30 seconds switch again. You don't need to attack the other monster for 30 seconds before switching back, just a quick click back and forth works fine.
A tip would be to just have the mage hit you back, getting some defending as a mage isn't exactly a bad thing.