[PyMOL] Sphere display conflict with screen selection

Dear all,

I am running pymol 1.6 on a Debian wheezy machine. I encountered a weird
conflict of sphere display and mouse selection. When I show the ion in
sphere, I cannot select residues by clicking it with mouse. Only after I
hide the sphere am I able to accomplish click selection. Does anybody have
some clues?

Thank you so much in advance!

Sincerely,
Chen