Antibodies that work in western very often do not work in IHC or flow as westerns often require that the proteins be denatured (although not necessarily always true). Since in IHC and especially flow we are looking at native (folded) proteins this can often cause problems. An antibody might be generated by a supplier because it specifically recognizes an antigen that is exposed when a protein is denatured. This antigen may be difficult to detect when the protein is folded in it's native state much less then fixed in a cross-linking fixative. (and vice versa for IHC specific Abs).
I find that when companies say it works in western without mentioning that it does not work in IHC, it usually means they have not tried it. IF you are desperate you can try it but it may or may not work.