I notice during sex I can't cum! What could be wrong?

Orgasm. This is called anorgasmia. They are a number of possible causes. A common one is medications, specifically the ssri or snri type antidepressants. Examples: Prozac, effexor, (venlafaxine) cymbalta, lexapro. Other meds can do it too. If not taking medications, further evaluation for metabolic or hormone changes. Then, psychological/physical. Anxiety, upset, depression. Speak to your md for evaluation. Referral.