There is a problem with my clone stamp strokes. They become invisible under a lot of different circumstance.
When I adjust the transformation of a clone stroke it becomes invisible.
However, even when I don't adjust the transformation, they usually become invisible anyway... not sure when or why... but I frequently do clone paint work... only to find that the results have disappeared.
The strokes are still present in the Paint effect on the layer controls... they just aren't visible on screen.