I've noticed the same on checkpoint mode, it often happens, I think, after an objective is captured. Renders a lot of the weapons useless. It's pretty bad, especially if you've just respawned and need a replacement for the bolt action in hardcore mode, the downed weapons' time should be extended.
If you observe closely, you will notice that the weapons you cannot pick up are the ones that were available before you resupplied.
That is the issue here - the system will not let you resupply and then pick up "old stuff" ...
A mutator or .ini variable would be great for this.