As Doug Yanega pointed out, you need to have enough bees per acre to do the
job, and get rid of any competing bloom (If you can.  If you can't, increase
the hive count to compensate).  We tend to put in 3-4 hives per acre, but we
have put in as many as 10 hives per acre for some growers.
 
If the bees are pollinating a hybrid carrot, one of the issues is the flower
itself.  Apparently, some of the carrot hybrids are not recognized as carrots
by the bees.  The flowers just smell too different, or look too different, or
something.  So when you have a field, or a cage, of two varieties of carrots
that you want to cross-pollinate, the bees only work one variety or the
other.
 
My husband went to a Western Apicultural Society seminar at which Eric H.
Erickson of the U.S. Dept. of Agriculture discussed pollination in general
and carrot pollination in particular.  I have seen five of his articles on
pollination; four of them are specifically about hybrid carrots, and the
fifth is about hybrid seed production in general.
 
Here is the abstract from an article he wrote (with C.E. Peterson and
Patricia Werner) titled "Honey Bee Foraging and Resultant Seed Set among
Male-fertile and Cytoplasmically Male-sterile Carrot Inbreds and Hybrid Seed
Parents" for the _Journal of the American Society for Horticultural Science_,
September 1979:
 
"A study of foraging by honey bees among cytoplasmically male-sterile and
male-fertile seed parents of carrot revealed that honey bee discrimination
between the fidelity to carrot phenotype and genotype were evident and often
extreme.  Some lines were extensively visited while others were virtually
ignored.  Wide differences in seed set were evident among male-sterile F1's
and inbreds and male-fertile lines.  Differences in seed yield were
correlated with foraging preferences, but the quality of nectar from the
stomachs of bees was not."
 
I don't know if Eric Erickson has an e-mail address and I don't know his
phone number, but you can reach him at:   U.S. Dept. of Agriculture,
Agricultural Research Service, Honey Bee Research Unit, 2000 E. Allen Rd.,
Tucson, Arizona  85719-1596.  Unfortunately, the honey bee research unit may
be closing at the end of September due to government budget cuts (Call Your
Congressman!), so this address may not be valid for long.
 
I don't know if unattractive hybrid carrots are the problem, but if the bees
aren't working the carrots (and there are otherwise enough bees), you can
mention it as a possibility to your grower.  If you want the titles, etc. of
the other articles, e-mail me, and I can send you a list.
 
Good luck!
Shawna Roberts
Gypsy Bees
Hollister, CA