I use clamps around the tubing with the stems inside and it makes for a good system without the mess of Vaseline. Sometimes I have to stick padding in the clamps if the stems are too small. They worked well. So I think it's valid to think that the stem was losing was losing water too fast, resulting in stomates closing. The syringe allows you to adjust water level in the pipette easily.
Some brands work better than others.
Before it dries place a little scotch tape over a small part of the wet polish so that you have a tab to pull on. If you determine the surface area of the field of view and the surface area of the leaf, then you can determine the number of stomates on the leaf. Or if you just determine the surface area of the field of view then you can determine the number of stomates per cm2. Count several fields to get a good sample. To open them again, replace with distilled water.