Nothing obvious Chris,

MoI hasn't got the strongest filleting engine out there so it does struggle to solve some fillets on occasions.

Your model could do with a little cleaning up, but on doing that the fillet still failed, so one work around would be to cut the model up using Boolean>Diff with a couple of lines as shown in the picture, apply the fillets then 'glue' everything back again with Boolean>Union.



Cheers