Are u talking does her pussy get wet before you enter or does she get a sweaty, wet workout after the deed?
Either way, you are not making her mentally horny and receptive to your dick. You can use KY jelly to lube her pussy, but if she is not mentally receptive to sex with you, she will be just be going through the motions like a dead fish, and not be wet and sweaty after the deed is done.
Like NLion's suggestion, read the wikipedia about foreplay and figure out how to 'press' your girl's sensual buttons to make her lust for sex with you. Once she gets used to thinking about sex with you, she can become instantly 'wet' with the right verbal suggestions or physical non-sexual touch.
If all fails, your girl has either entered menopause or she has some major hangups with regards to sex. She may have to see a counsellor.