Maybe the X parasites somehow damaged the grapple beam in a way so that it only worked at point blank ranges, couldn't be fixed and couldn't be replaced?
Er...maybe the gun is magnetic, and the bar things have a bit of magneticity...or whatever in them so that the gun can stay on for about 2 seconds. That or they're kinda like semi-circles, so the gun can stay on it while Samus grabs the next one. It'd be easier if I had a picture...