I've seen lots of cotoneaster covered with berries this autumn, and already being polished off by the blackbirds. Did yours flower this year?
Mine is laden this year too - sorry, that doesn't help much
C. frigidus is my least flowering/berrying cotoneaster every year.
I think 'Cornubia' is a hybrid or cultivar of this and never fails to perform.
Fortunately there are enough C. lacteus berries to supply the birds as well
Maybe the flowers got damaged by frost