Dahlias

I have a dahlia problem -
I have 3 different varieties in pots that stand in a row on the deck and something keeps chopping off the stems of the middle one at soil level.
There is no damage to the leaves, no evidence of a snail/slug trail on the pot or top of the compost and I've checked for snails at night. I moved the pot up onto my potting bench and it was fine for 2 days and then this morning another severd stem. The damage to the stem is a tiny version of the shape left by beavers when they fell trees! I did find a lone vine weevil adult on the deck, but again the damage doesn't look like vine weevil damage - no notches on the leaves.
On the plus side I'm getting an awful lot of new cuttings ...
Cheers all.

Edit - and a root around just below the soil didn't find anything, and there are lots of fresh new shoots coming through which would make much better eating!