Problem setting up criteria based sharing rule

I am trying to test set up of a criteria sharing rule in sandbox ; I have set this to be where the object record was last modified by me.  I have created a custom public group and put one user in it.    However, there doesn't appear to be any change to the record visibility when logging in as that user; banner alerting to background processing appearing but operation completed.  Have refreshed. I have checked OWD for the object and Internal access is set to Private.  Not sure what else to check, please can someone advise what else?    Thank you in advance!