You may be getting the tube trapped between the rim and the edge of the tire. That prevents the tire's bead from seating properly under the rim's "hook" edge, AND puts the tube where it'll be trying to heave the unsecured bead over the edge of the rim too.

To prevent that, after getting the tire and tube onto the rim, squeeze the tire together so you can see down into the rim bed. You shouldn't see the tube at all. If you DO see the tube, knead and work the tire & tube with your fingers to coax the tube up into the tire casing where it belongs.

Another "gotcha" is when the thick rubber base of the valve is down in the bed of the rim. It can keep the tire's beads from getting under the hooks, too. To prevent that, after getting the tire & tube onto the rim, push the valve up into the rim so the base is definitely up in the tire, not down hogging space in the rim bed.

Lastly, pay attention to the bead line on the tire. Give the tire 30 PSI, then take the pump off and spin the wheel while watching the bead line on each side of the tire. Does it have high spots? Does it have spots where it ducks out of sight below the edge of the rim? Let the pressure down to "very squishy," pinch it, look for the tube being where it doesn't belong, then even up the seating with your hands and try going back up to 30 PSI. Then 60 PSI. Still evenly seated? Keep going.