Yes the outer ring on a stock balancer can definately slip. If that turns out to be what's up get rid of it before it pitches the ring off.
To check actual top dead center location in relation to your mark you can do the following.
Buy or make a spark plug TDC finder tool. It's simply a sleeve that screws into a spark plug hole with a center stud that screws in and comes into contact with the piston. It's not that tough to make one out of an old plug, I've made a couple of em.
Then turn the motor over carefully with a long ratchet or breaker bar until the piston contacts the stud. Mark it on the balancer. Then turn it the opposite direction and do the same. TDC will be half way between the marks. Don't get carried away and punch a hole in your piston, gentle here.
You can also ballpark it by looking in the plug hole doing the same drill if you can see the piston come to the top with a penlight or scope.
Hope that helps. May be way more info than you need