A common mistake.
2 things to keep in mind:

An IF statement, loops only once, IF the conditions match. (Duh :P)

A WHILE loop, loops infinitely until you break the loop within the loop.
OR you can also make a condition in the WHILE so it keeps looping until the conditions don't match.

In your case, keep looping (=> keep checking the keys) as long as the left SHIFT key is on.