It sounds like someone might at one time have tried to rack the slide with the lanyard loop lock out (LLLO) engaged. If the slide is racked, or attempted to be racked with the lock out on the hammer strut will bend. And since it only has one direction to bend, toward the magazine well, it goes in that direction. That in turn caused the spring to bend in toward the mag well and that puts pressure on the mag and does not allow it to drop free. So the only solution is a new hammer strut. HK knows of this problem and will fix it for free but the real solution is to turn off the LLLO and never use it again.
The hammer strut is very thin so I do not recommend trying to bend it back though it has been done successfully. The risk is you make a hairline crack in the strut and it ends up breaking unexpectedly and that could cost you your life if this is on a duty or carry weapon.
The other problem that causes this problem is that on top of the LLLO there is a small recess that the base of the hammer spring fits in. If the spring is not down in the recess but rather caught on the lip of the recess, that causes the spring to bend also. The cure for that is simple enough. Remove the LLLO and make sure when you put it back together that the base of the hammer spring is fully in the recess. That happened to a Tactical I have and getting the spring back down in the recess where it belonged fixed the issue once and for all.
Since it is happening intermittently there is one more cause. You are getting off the mag release too rapidly and not giving the mag time to fall out. If I were a betting man this is the cause I would put my nickel on. You must depress the mag release long enough and hard enough to completely disengage the mag and give it time to fall out. Since this is happening only in your speed drills then this is where I suspect the problem to be.
