It is quite likely that the hand was never pressed all the way onto its pinion at the factory (there is little or no QC in rep manufacture) & the act of uncasing the watch may have been enough to loosen the hand to the point where it is now too loose to stay in place. Your only option is to uncase the movement again, remove the minute & second hand (if fitted), press the loose hour hand down (GENTLY) with a clean toothpick (I assume you do not have a proper tool for setting watch hands) until it gently seats & reinstall the other hands.
IMPORTANT - Before recasing, pull the stem out to the time-setting position & manually run the hands through an entire 24-hour cycle to be sure none of the hands are coming into contact with each other, the dial or the inside of the crystal. Check, also, to be sure the date-change works correctly. If everything is working, press the stem back into the winding position & check that it also winds properly. If all is well, recase & take pride in having repaired the watch yourself.