your ground check may also be a problem, maybe you are not always considered grounded when you should be.
As for input loss in fixed update, you can check this link https://forum.unity.com/threads/check-for-user-input-in-fixedupdate.214706/
Basically, inputs are updated before each Update(), and depending on the computer, it is possible that many Updates occur between each FixedUpdate. If you check in FixedUpdate, sometime you will never "see" the key being pressed, because the key press was only true during one Update, and was erased by another Update that occured before FixedUpdate.
What will work is something like that https://pastebin.com/nS1cuFLq