There have been a number of tones recently where i can't get it up when wanting to be sexually active with a girl. why is this?

Many causes: Having ED is very common and the incidenec increases with age. It can be due to physiological factors or psychological causes. Physiological causes include low testosterone level, diabetes, and thyroid dysfunction. A simple blood test and physical examination can usually identify the problem and allow appropriate treatment.

Many potential cause: Also consider performance anxiety, newly developed tension in the relationship, newly started medications, or alcohol use. It could be that the sexual experience has grown staid or repetitious as well. If she appears to be enjoying it less, this could contribute to anticipatory anxiety, less satisfaction and performance anxiety. Sometimes these problems are intrapsychic in origin - self-sabotoge.