1) Wait. It will eventually get to 32, but for how long is anybody's guess. Maybe a minute, maybe more, maybe less.
2) Increase your Max Connections if you want more than 31/32. However, I've seen that it will just as often stay one under even then: 37/38 - 41/42, etc.
3) Go to https://bitnodes.io/ and find some juicy nodes to add to your list using the addnode=IPADDRESS:8333 command. I've found this will usually max out my connection choice: 32/32 - 42/42, etc.
Cheers!
The default Outbound connections a Bitcoin Core node will make is 10, sometimes 11. This is hard coded into the application. You can force it a little higher with the addnodes command.
All the rest of the connections you have are Inbound connections.
31/32 means you have 10 Outbound and 21 Inbound connections.