Clutch not engaging properly

This could be the slave cylinder but it could also be the throw out bearing, or it could be a large hydraulic system issue. Here is a guide that will help with this this:

https://www.2carpros.com/articles/clutch-doesnt-work-sometimes-or-not-at-all

However, if it were me I would just go ahead and replace the entire clutch system if you have not done so in the recent past. I am attaching the process for this which is not detailed so we can get more detail if you need it. Let me know what questions you have. Thanks