Form submitting issues

Hi! I’ve custom made a subscription form for my clients, using the Wix editor. It works fine some of the time, but more often than not my customers complain that they get an error (“form wasn’t sent”). I’ve tried it once and I got the same error, with a line of code at the bottom of my page (which my clients don’t get, I was in preview mode). Unfortunately I did not screenshot it. Every time after that, and I’ve tried many times, the error did not recur and my form went through without problems.
It’s impossible for me to pinpoint when a client gets an error and when they succeed. Can anyone offer any insights as to how this can be resolved? Needless to say, this is hurting my business.