The agreement with the CTO was to have a raise after 6 months. There was no previous agreement of a specific number, just a "raise".
After 6/7 months I went to talk to one of the founders about that specific issue, i.e. that I was promised a raise, and that I wasn't expecting anything below 20%. I came to know during the time that I work there that was being paid well below my coworkers (and the market rate in general for my position).
Their response was that I would only get raised on the next company review (still some months to go, but note that I was promised a raise after 6 months), the raise would be around 2%-3% (note that I said to them, quite direct, that I was being underpaid, at least 20%), and the project manager even said that I would be subjected to a performance review before they would give me a raise.
So they failed me at least with 3 things: no raise after 6 months as agreed (still some months to next review), would not raise me enough so I would still be underpaid, and I could even not get raised because I would have to be reviewed first..
Basically, I quit on the next working day, and one or two months later, most of the team got laid off, so there was bigger problems on that specific company..