Recuva scanned my flash drive for 33 hours and at the end it showed zero files to recover.
Unfortunately, the bottom line is, that means there probably isn't anything that Recuva will be able to recover. Flash storage, such as used by USB Flash drives or SSD can often be more difficult or even impossible to recover from, depending on how aggressive the control circuitry for it is in trying to optimize the free space for write speeds.
If you switch to Recuva's Advanced Mode (using the Switch to Advanced Mode button in the top-right corner), you can click Options > Actions to access a small number of options regarding how Recuva displays scan data; normally the software filters out a small amount of typically undesirable files so as to avoid cluttering your view. You can find out more here: https://support.piriform.com/hc/en-us/articles/360048410512 but in short, turning on all of the Scanning Actions except 'Deep Scan' will prevent Recuva from filtering any files out.
Speaking of Deep Scan, it sounds like that's what you used. Please note that this option is only suitable for certain, specific file types, namely those enumerated here: https://support.piriform.com/hc/en-us/articles/204044074 - if the files you are looking for are not of any of these types, then Deep Scan will, by design, not detect them. You'd want to use a normal scan in that case.