Tiny capillary defect in sealed tube

Hi all,

I have just begun to try some simple glassworking by trying to make sealed tubes from cheap glass "eyedropper" pipettes. I am using a Map-Pro gas flame (Propylene gas I believe). It seems that every time I try to seal an end by just rotating the tube in the flame and letting it "ball up", I end up with a tiny, tiny little capillary between the inner and outer surface. This persists even if I heat the end up hot enough to begin to drip and then let it cool in air again. Anyone seen this before? What is it called? How do you prevent it? What causes it? I have attached an image from under a microscope. The entire image shown is about 6mm across. The channel must only be a few microns in diameter. I have tried to adjust the colors in the region right around the channel to make it more visible. It joins with a pronounced funnel-like shape at each end where it meets the surfaces.

Thanks for your help!

I'm not at all knowledgeable of glassblowing, but I have had a similar issue a few days ago and I've had time to consider the reasons. I believe the main cause of this is low flame temperature, or at least a slow heating Δ, which results in some modified characteristics of the glass. So, even if you can reach sag or drip temperatures, maybe they are not sufficient.
The other guess is that the glass may not be cleaned up well, and residue from burning/pyrolysis on the glass prevents it from coming together.
Like I said, I am not well versed in glassblowing, although I find it one of the most challenging and beautiful crafts, and only for a lack of resources have I not begun learning in, apart from sealing capillary tubes