Proper tubular tire installation
If you have got a loose area of tyre next to the valve, you need to 'distribute' this looseness to some other part of the rim.

Put a screw-driver across the rim under the tyre, and roll it around the rim away from the valve.   The screwdriver will slip on the metal rim, but grip the tyre, so it will revolve.

Do this for both loose areas, each side of the valve, before you blow the tyre up.   Once up to pressure, the tyre should run true, and be well stuck.
Cheers...  Pete