How to make the test ball seal better

The new 4" test ball is letting water past, very slowly but enough to invalidate the hydrostatic test. I can see every joint but not the test ball (it's in a sealed wye) so I know it's the test ball. I've drained and repositioned plug three times. It's a little better but still leaks slowly. So what do you think of slathering some rectorseal or something else on the plug before inserting? Might harm the rubber? Any other ideas?

If you slide it in far enough to be inside the pipe, not a fitting, it will seal when you use enough air pressure.

These are nice

solved

Forgot I hadn't posted that the problem was solved. There was a very slow leak at another joint (the only one that touched a bit of dirt hence it took awhile to find it). Job approved and done. Thanks to those who responded!