Pharyngeal pumping rate measurement!

Hi everybody,
I’m interested in measuring the pharyngeal pumping rate. What I have been doing is: 1. make an agarose pad, 2. suspend few L1 animals in M9 along with some OP50, 3. digitally record the pumping under DIC at 60X. My problem is most animals just do not pump. Its true even with N2. I do not use any azide or levamizole with the M9. Does anybody got any suggestions? Thanks in advance.