| Cover comes up a lot in our games, at least once per combat and probably about 3 times per combat. With the significant amount of movement possible in the game, cover is easy enough to obtain. It's a sure thing for any ranged attacker. If you don't see cover used, then it's because a/ you have no ranged attackers on either side, b/ the DM doesn't provide interesting terrain, c/ your ranged attackers are not targeting the bosses behind the enemy lines (allies provide cover) or perhaps the DM is not arranging the bosses behind mooks.
My guess is simply a failure on the DM's part somewhere. For instance, the standard modules provide a huge number of possibilities for cover so I'm sure you're not using them (which is fine, but it does provide an example of interesting terrain).
Concealment isn't as often and depends on the PCs chosen. If you have a warlock or rogue or stealthy ranger, you'll likely see a lot of concealment. You still won't see as much concealment for opponents, but I would think you should still see some, perhaps 1 in every 4 encounters. This is also mostly encounter design by the DM (e.g. shadowy rooms, encounters at dusk or night, stealthy opponents, magic).
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