Keyboard retainer issue

I was trying to clean my MacBook and apparently something got under the keys, which meant I had to take a few off to clean them. When I put them back on, they were all working bar the left shift key, which does not seem to respond. I opened it today to see that the retainer (I imagine that's what the bit under the actual key that is not the button is called) is loose and can be removed easily. I tried putting it back on but it hasn't worked so far.

I am sure this is not a case of water damage as I tried poking the bit under the button with a pen and it did make contact.

I know this isn't a major issue as I have remapped my shift key to the function one, but I'm very uncomfortable with the current set-up and it looks like only a minor tweak is needed to get it working.

Any help would be appreciated, thanks in advance.

I assume you closely inspected the tiny tabs on the "retainer" piece as well as the tiny metal spring tabs that hold it in place? Perhaps a small adjustment to the spring tabs is needed. You may have bent them slightly when removing the key.