I have a 2000 Volvo S40 and the left lowbeam does not go on unless I tap the fusebox, located right above the hood pop. I am wondering if the problem would be with fuse 10a, 15a or 20a? Or if not those then what?

Check the relay switch. It is a small square box about 1"x1"x1" and when you pull it out, has 4 or 5 terminals depending on the style. some are long rectangles. They are located either in the fuse box or in another box specifically for the relay. Or you can do a search on google for headlight relay location. Also, try wiggling the fuse or better yet relace it. there should be a separate fuse for the right and left headlights. I am betting on the fuse because the headlight relay should control both lights, whereas each light has its own fuse. Hope this helps.