I've never tried this kind of thing before but a mechanic seems bugged. I had a tough persuasion check (30) and my character had only one bonus. Guidance wasn't enough to get me to 30 if I rolled a 19 so I looked for other options. I thought I'd get some kind of bonus if I charmed my target but it didn't help at all. I never use Charm for conversations so I don't know if that normally helps. Possibly the normal effect is negated because (avoiding spoilers) the outcome of the convo was very important to the plot. If that's the case it still seems unfair. Took me like 8 reloads to roll a Nat 20.

Also, venting a little, charming that character was only 1% easier than rolling a 20.