Occasionally there will be a problem with mallets - after all, that's where the action is. Mallets take a real beating, right along with the marimba bars that are the brunt of the bonking.
I'm not sure how this pair of mallets got in this condition, but it's an easy fix. As long as your mallet heads are not cracked or somehow disintegrated, then they are mendable. I'll put a spot of glue in the grooves on the sticks and force the heads back into place. Generally the depth of those grooves keeps the mallet head from shifting, but in this case, not so.
Now cracked sticks are MUCH more common, as wood is easier to bust than mallet heads made of spage age polymers. Young malleteers tend to display enthusiasm and vigor and so things happen. Masking tape seems to be the fix of choice and lasts about as long as the next tune. So if your sticks crack, send in the kindling that is still attached to the mallet head - it's easy for me to punch out the damaged stick and replace. Voila! New mallets.
This goes as well for the mallet heads on your old old "Stacy" Sticks, if you are lucky enough to possess them. Never throw those away, they are primo heads turned individually on a lathe by Stacy Sabella, who is no longer in business, as far as I know.
So hang on to those broken mallets and replace the wood.