external monitor macbook air (early 2014) not recognised

I have serious trouble connecting my external monitor. I have tried resetting using Option, Ctrl, Shift and power button. Sometimes this works, but not always. I have also tried to detect external monitor with Cmd Option, but monitor is not found. I especially have trouble after my Mac has gone to sleep. Restarting sometimes helps, but again not always. I am using a thunderbold to HDMI adapter. The thunderbold connection seems to be fine, when I use it for cabled internet, it is picked up immediately.

I hope someone has some more suggestion as to what I can do.