This can be simply down to the fact that sexual interest tends to ebb and flow over time. It can also be related to specific issues in the relationship or external pressures from outside it. Anxieties surrounding sex can also come from different expectations about how much sex you think you should be having. They may also need the setting and mood to feel right. The best way to do that is to talk to your partner. You might find it helps to take the approach that we use in sex therapy.