Hey, paolo!
There's three things that, in my experience, could cause subtitles to be shown in a faulty manner, mainly on websites, since most or all players such as VLC are able to ignore little irritations:
1.) As SmallBrother has already mentioned: the encoding. Personally, I haven't had any issues so far with UTF-8 and I always recommend it, due to it being the most compatible and best universal one. Alternatively, you could try encoding it in ASCII, which is basically the 'standard' in the US and is being used quite often in Australia as well. ASCII can't handle special characters all that well, though, so if your error happens not to be due to encoding, I'd recommend you to stick with UTF-8; it's more or less a safe bet.
2.) The numbers. I've had quite a lot of annoyances with this one already. If you're on Microsoft using a subtitle editor, you'll be most certainly just fine. But when on macOS, most tools start the numbering at 0, rather than 1, for whatever reason. This could cause some sort of syntax error on web-based players, rendering the subtitle useless. You can check whether or not this might be the issue by simply opening the .srt file with a text editor of your choice and check whether the very first number is a 0 or a 1. If it's a 0 and you're on macOS, the easiest way to fix this is to open the file with
OS FlixTools, drag the respective file to the 'Synchronize subtitles' tab, set the proper input and output encoding and simply save it. This way, the numbering will be edited in order to start with 1 and you should be just fine. I specifically recommend FlixTools here, not only because it's a tool of our own, but after trying this workaround with 6 different sub editors on macOS, I found it to be the only one that automatically corrects the numbering to start with 1.
If you're on Windows, though, I can't really help you out on that one. You might want to try out a couple of subtitle editors and check if they either have a tool to renumber the subtitles, or just re-export them, hoping that the numbering will be fixed automatically.
3.) The line format. This one really sucks and it's a pain in the ass to fix it, since it's a classic compatibility issue.
If you've exported your .srt file on Windows using any tool, Windows applies its standard way of formatting lines to it, called CRLF. This is a combination of CR and LF.
If you've exported your .srt file on a legacy version of Linux or a macOS version older than 9, the line format will be CR.
If you've exported your .srt file on a current version of macOS or Linus, the line format will be LF, which I consider to be the universal standard to go with nowadays, since Microsoft's CRLF approach is wholly unnecessary.
Now, what does all this mean? Since most people use Windows, I assume that this web-based player and subtitle plug-in are primarily aimed to please Windows users, having no issue whatsoever displaying CRLF-formatted subtitles.
But, if not on Windows, it might be that the player only shows either the very first line of your .srt file, or no line at all, due to it having issues reading the line formatting on a code basis.
I can't really help you out with this one, since I simply kept avoiding websites that require me to change the line formatting (I'm on macOS).
But your best bet would be to just google for ways to 'convert LF to CRLF' or something like that. If I remember correctly, there's a couple different ways of achieving this, one consisting of compiling a version of Perl and building a script that handles the task.
I hope I could shed some light on your issue and, if you're able to resolve it, please let me know what you did to achieve this- for further reference.
Cheers,
kaze