Comms are for when you wish to indicate that you liked the player for some reason. If you spend the instance critiquing another player, particularly in a way that they dont find helpful, you probably arent going to get the comm, surprise surprise. By the same measure, just being awesome and carrying the group MAY get you a comm, if you arent an A hole about it. For example, if the healer is good but is also a jerk about it (like being pushy or generally non communicative), I might hand the comm to a dps who I thought was earnestly trying hard, even if they weren't very good.

If you are super concerned about getting comms, which tbf dont mean much, I suggest communicating more. Also dont be that healer who literally asked for the comm after saying nothing for the entire instance