Yeah, unfortunately, not every error or omision invalidates a fine. For something as simple as not ticking a box, when the box that should have been ticked is obvious because of other info on the fine, for example, you know that failing to stop at a red light is not going to be a maritime offence, then you are not going to be able to get out of it based on that.
If you believe you have a valid reason that your fine should be cancelled then your best bet is to use a service like
Fine Defender. I have used them twice myself and their service is awesome.