Well, it is as the title of the thread states: several subs work properly, but some don't appear at all. In Subtitle Workshop I can't open any of them, it says it is a wrong fromat or an unsupported one (while all are srt-subs).

Any ideas on how to fix this?

edit: found out some subs were timestamped very wrong, some start at 3 hours in, some at 6 hours in. Weird.

Solved by closing Subrip everytime a sub was finished and reopening it for every new sub. A bit of a hassle, but I couldn't find a faster way to make it work.

isn't there supposed to be a button where you can reset the timestamp?

Yes, but that one didn't work this time

I have also to close subrip each time i rip a sub.

Sometime subtitles are in format Unicode instead of ANSI

All of the subs I ripped yesterday were in Unicode.

My first idea was a possible character encoding issue: like ASCII / UTF-8, ISO8859-1 etc.
I don't use subrip - only used it once to extract the hardcoded subtitles from Angel Negro. It's a cool application.
Under Linux there is "Subtitle Editor" which does a good job. But it fooled me once by omitting multi-line subtitle entries.

Subrip has a few little glitches, so I won't be surprised if you had to close it before creating a new subtitle.

If the file starts with the hexadecimal values of EF BB BF (called Byte Object Marker) it is UTF-8 encoded.
This is the easiest way to check for a file being UTF-8 (best Unicode format if you ask me) encoded.