Try using some lube, also if you are taking certain medication such as decongestants, it will decrease the wetness. There is nothing you can do to change it, it varies.

The same exact thing happened to my ex. She used to get REALLY wet, I didn't even have to do that much to make it happen. Then she began using the birth control ring, and we had to use lube from then on out. It didn't matter what I did. She could even please herself with her vibrator, and she still wouldn't get wet.

It might be that you're slightly anxious or worried or something without realising it?, that might cause you to dry up or not get wet, try and stay relaxed