Same problem here...I think it's a 3 month old thing LOL
Nala would rather play with her toys...but if my hands cross her line of sight...she will definitely try to mouth me. I've learned (at least with her) that she is more interested in my fingers -- If I see her eye'ing my hands, I will make a fist and keep my hands still. Usually she will divert her attention to something else when she no longer sees 10 wiggling "play things". If she does go after my fists, I will cross my arms Indian style, stand up, and turn my back to her. This has worked for me on and off....the last time I stood up and turned my back on her she started tugging at my pant legs LOL One step at a time...if I find this stops working, I will leave the room all together.
I too tried ALL the recommended approaches to mouthing, but there was no textbook answers for me. I've just had to try different approaches and see what she reacts to. If I pull away, or try to force her snout closed she gets 100 times worse...I've tried to redirect her attention to a toy, but the action of me reaching for a toy with my hands, almost intensifies her nipping, since my fingers are moving, I'm pulling away from her, and I'm going after a toy. So right now, the stationary fists are the best I've come up with.
We are slooooooooowly getting better with the mouthing, however, if I try to hold her in my arms (when she wants to play) she will leap up and try to eat my hair....which sometimes gets mistaken for my chin or nose...if you think hand nipping hurts...try the nose!

LOL. I correct her with a toy, but it's usually hard to divert her attention until I put her down.
One day at a time
