Been feeling dry

Hi everyone, I’m 18 years old and have been having sex with my boyfriend. Lately I’ve been feeling a bit dry down there while we’re having sex and don’t know know why. It’s not that I’m not attracted to him or that I’m not turned on because in the beginning I always am soaking wet but then when he switches position to the back I feel dry? Also when he uses his fingers on me it doesn’t get me wet at all despite it feeling really good and me feeling turned on. Any suggestions or explanation as to why this happens?