Is this normal?

If this is one of those trumpet style horns, yup.

I'm on my third one in eleven years.

The trick is to be sure you mount it so that
water will drain out of the horn. I get sal****er in mine once in a while, even
though it's on a rack on the top of the pilothouse. It seems like if the water
is allowed to sit on the noisemaker long enough, it cruds things up and the
horn doesn't blow. This third horn has a very deliberate slope to it, in an
attempt to insure that water won't do it in like it did the two predecessors.

The other issue with a horn can be poor electrical connections. An electric
horn needs a good jolt to blow, and won't work well if wires are undersized or
the connections are crummy. Sounds like you
eliminated that possibility by hooking it directly to the battery.