> Maybe the product is not as effective against N. Ceranae

Actually, recent work shows that treating N. ceranae with fumagillin makes it worse.

QUOTED:

Use of fumagillin may *increase* the prevalence of N. ceranae
and is potentially a factor in replacement of N. apis by N.
ceranae in US apiaries.

Spore production of both Nosema species increased at
lower levels of fumagillin residue, although not significantly for N.
apis, and N. ceranae spore production was double that of N. apis. 

We demonstrated that very low levels of fumagillin residue, possibly
below the detection limit, affect the interaction between the
microsporidia and the host. The higher number of N. ceranae spores
produced in treated honey bees could potentially increase the
pathological effects and transmission of the microsporidium

The risk of contamination is significant:

Although the US Federal
Drug Administration disallows fumagillin usage during the
foraging season, marketable honey in U.S. was found to contain
60 ng/g of fumagillin residue

Fumagillin is not a safe product for humans:

Bicyclohexylammonium fumagillin, an antibiotic isolated from
the fungus Aspergillus fumigatus, is toxic to mammals and must be applied
seasonally and with caution to avoid residues in honey.

Huang, W. F., Solter, L. F., Yau, P. M., & Imai, B. S. (2013). Nosema ceranae Escapes Fumagillin Control in Honey Bees. PLoS pathogens, 9(3), e1003185.

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