Originally Posted by
Nerull
You haven't mixed decimal and fractional 26" rim sizes have you? They're slightly different. What size does your tire say?
+1, look up
ERTRO rim sizes and see if that may be the issue. Otherwise it's simply poor seating. If the tire is low in one area, it'll be high elsewhere and blow off. This also happens if you trap the tube under the tire where you last push it over.
I always start with the tube inflated to shape, and end at the valve. The penultimate step is to push the valve in to draw any trapped tube clear, then pull it down to settle the tube over the tire bead. Lastly, inflate to 10psi or so, and spin the wheel to check that the reference line molded into he tire is the same height all the way around.