Have a look at the transaction outputs, specifically the one highlighted in YELLOW (this is the change output)... if that is very small (or doesn't exist), then you won't be able to increase the fee
Small correction here. If your transaction has no change outputs you can still bump the fee. The money will come from the remaining outputs and yes that means one or more recipients will get less than you intended. Electrum does it like this because if, in a hypothetical scenario, you're sending your entire wallet balance to an exchange you can still bump the fee.
If your transaction does have change outputs electrum will limit itself to those when bumping the fee.
Credit goes to SomberNight Electrum dev for explaining this to me.
