Macbook Air trackpad not clicking

There are several reasons why the trackpad stops clicking - a swollen battery is one. If the battery has swollen enough to affect the trackpad you should be able to see it. Put the computer on a hard flat surface. If the computer doesn't lie flat on all four feet that could indicate the battery has swollen. Move your head so it is at eye level with the edge of the case. If you can see a slight bow or if the lines of the computer case look flexed that is another indication. If you suspect that's the case unplug the computer and turn it off immediately - then head to an Apple Store or authorized service center. The problem will only get worse and do more damage.

The trackpad has several screws holding it to the case and these can be tightened or loosened to change how the trackpad feels. After a few years my 2009 MBP began to intermittently fail to click and I took it to my local Apple Store when I happened to be nearby. The genius adjusted the screws and didn't charge me even though it was out of warranty. Not saying you can expect the same but it took him less than 15 minutes and he gave the computer a quick looksee at the same time.