We consider a "good seal" to be a 360-degree connection between your breast and your flange. There should not be any gaps between the flange and your breast.
What to do if you're experiencing breaks in the seal?
Loosen your bra band and/or straps
Try a different bra type with a bit more space or less compression
Adjust your breasts "out" (i.e. away from center) before inserting
Check bra band placement is on the neck of the flange