When the reticle becomes a dot, the button operation does not work.

Yep.
Been reported many times.
It happens when you attempt to throw a med-pack or ammo-pack before the animation is finished from throwing a previous med/ammo-pack, as in spamming the heck out of it.
Switching to pistol/gadget, then back to your primary will fix it.
Pain in the butt when in the middle of an action packed encounter.