Hi Paul,
That happens with images where most of the pixels have values around 0.5 (half-way between black and white). When this happens, the cursor is difficult or even impossible to see due to the way cursors are drawn in PixInsight. Note that if you click the mouse, the cursor normally is visible (although this is only a partial solution).
The best solution in these cases is applying a Screen Transfer Function (STF) to lower the levels a bit, just enough to see the cursor again.
