In IHC an enzyme proceeds the substrate-chromogene and it converts a certain amount of chromogen in relation to the bound primary and secondary and in relation to duration of incubation.
In IF the signal only relies on the amount of the bound antibodies that carry the fluorochrome.
So the kind of producing a signal is rather different and it is rather logical, that the antibody-titer can be different.
It depends also on the applied method. one-step, two-step, three-step. Every step leads to a further amplification of the signal.
I would try to prolong the antibody-incubation and to increase the antibody-concentration, until too high background.
good luck, gula