Investigate and fix problem with Analytics

I'm having trouble with our Google Analytics. It's showing a much lower number of sessions compared with our Ad clicks. (e.g. for the last 7 days, we had 198 Ad clicks, but only 16 sessions showing on Analytics - I've attached some images to show this).

I've tried to investigate, but just can't seem to figure out what's going wrong. It's probably something fairly simple or stupid, I'm really not great at this stuff!

I'm looking for somebody who can investigate this and let me know how to fix it.