I have a Royal Guard and no, this should not be happening. Make sure the switch contacts are clean and adjusted to make contact flatly. If it still happens, take a look at the relays that they trip, perhaps the activator plates were improperly lubricated or are dirty and have become sticky. My targets never miss a beat. I did have to fabricate a new bakelite linkage that moves them, due to elongation of the mount holes.
The 100 point bell on mine is crazy loud, how about yours?

Are any of the holes in the bakelite piece that moves the targets worn or elongated. If so maybe the targets arent always fully aligning completely with the activator switches? Its curious that it works every time by hand. Is it possible the target shift is happening too soon, before the target relay switch can close? Maybe gapped to close too early?

If memory serves, I think there is a target position relay (not quite certain of the name) under the playfield that changes states depending on whether the targets are shifted left or right. I'd also clean and double check the switch gaps on that relay, as it probably carries the signal to the target relays.

The problem with mine was that the elongated holes would cause the targets to not always line up with the switches, so not every target hit would register. You can visually check if the targets are always aligning with the switches behind them.