The last byte of the PID is the issue. It has to be below the gender threshold.
Snivy has a male/female ratio of 7:1, so it has a gender threshold of 31. If the last byte is 31 or greater, Snivy should be male, else female. Now your Snivy's PID is 11FB33F3. Take the last byte (F3), converted from hex to decimal that's 243.
Everything from 0 to 30 would be fine. So if you take 30 (1E in hex), the valid PID for Snivy would be 11FB331E,
You can check this for more detailed info about gender PID relations https://bulbapedia.bulbagarden.net/wiki/Personality_value#Gender