--imsi *** not filtering/tracking imsi's

[crt404]

So on a previous version, a couple of years ago (ubuntu 16.04), you could track imsi's with the argument --imsi=206. This used to filter any imsi beginning with that string.
Now, on the latest code, it comes back with nothing. I reloaded my old hard drive to check I was using the same command, but now the --imsi or -m option does not function.
Am I missing something?
For info, I'm running: sudo python3 simple_imsi-catcher.py --imsi=206 --sniff

The only obvious difference is that it now runs python3, which it didn't before. Any suggestions welcome (even if it proves I'm being stupid!).