If your glass is not really clean, you can get gaps in the paint line. You may also have areas where the paint beads up rather than flows evenly.
You need to clean as best as possible first. Then just before painting you can use a dilute solution of the paint to scrub all over the glass with your finger or other firm material. Wipe any residue off with a paper towel and you will find that the prepared paint will flow evenly onto the glass.