I just had this issue occur myself. I set up a settlement to be a pure military base, it currently has a level 13 military rating. I've deployed them before, many times. They are my hard-hitter for when those 100-pawn sieges show up. Which is what happened. I took a quest (protect 2 prisoners with blood rot for X days and get redonkulous amounts of plasteel) that included two attacks by 80+ enemies. They wound up both being sieges, and the first one I defeated by calling in my three smaller (Ranks 4, 4, and 3) settlements to hold the line while my noble dropped an aerodrome barrage on the enemy. It forced them to retreat quickly. No big deal. I recalled those squads and they all left. Military Cooldown event starts ticking over.

Then the second siege shows up. I call in my rank 13 force. They fight a hard, pitched battle against the enemy and force them to retreat. I send them away and they all leave. I think nothing of it.

Other settlements' Military Cooldowns tick over, and I pay no attention. I keep paying no attention until I have another random siege show up. I go to call my heavy hitters...and nothing. They're still showing as Available: False. I check the Event log. No Military Cooldown event. Think, "Huh, that's weird." Call in my other settlements and support them with my own pawns, manage to drive off the enemy.

I go back and check. Yep, they've all got Military Cooldowns ticking over. But nothing for my main base. I go into the Military tab and hit Reset Pawns. Nothing happens. So I swap my big military settlement's squad out for a single dude, pay the $160-odd to call in a second squad. He arrives. I send him off. He leaves. Still no Military Cooldown.

I've tried reloading. I've tried dropping out of Rimworld entirely. Nothing works.

For anyone looking for a fix, delete the squad template from the create squad menu. Immediately fixed it for me.

didnt work for me :(