When Muscari clumps are becoming congested they often 'come to the surface' and sit there 'greening up' like potatoes in sunlight. Were they mine, I'd dig them up, thin the clump out to big bulbs only, add some well-rotted compost - and replant.

I don't have that problem with crocus (apart from salzmannii); I do with Galanthus. They sprawl around under the trees in summer, looking abandoned, until I pounce and restore them to the depths in some amended soil. Then it takes them about three years to wriggle to the surface again. It's not uncommon...

I have had the same with Amaryllis Belladonna and they will stop blooming unless dug and divided. Al