I have a heavy in sniper armour, who wield a Hel cannon.
I got 4 ammo clips and used 2. Heavy now refuses to reload the 3rd one.
Check the amount in the clip. It could be empty. Or insufficient for a full re-load.
Thanks for the idea, but all remaining clips were full.
I`m talking about a reload during a mission.
No worries. I have had it occur during a mission that I had clips in my body inventory that I assumed were full, but upon looking at them they were either at zero or less than a full clip. And I couldn’t reload on the mission map. I think it’s a problem with how the +Ammo button adds more ammo by filling your weapon but then creating a partially filled clip. Anyway - there are lots of bugs that remain in the game and this reload issue is certainly one of them.
Yeah, but on missions there isn`t a +ammo button. My problem is strictly related to the case during a mission.
But I think, you fully understood my problem. 
Try to move ammo to a different spot in inventory and try again. It was working in Backer build…
Had the same bug, full magazine in backpack but couldn’t reload. Moving things around in inventory didn’t help.
It was a Sniper who also had proficiency in assault rifles, and he was carrying both and had spare magazines for both. I could reload the sniper rifle but not the assault rifle.