Be sure the skin, where you are going to apply the sensor, is clean and dry, free of any oils, body lotions etc. I believe one of the reasons for the alcohol wipes is to be sure there are no oils, even natural body oils, are present which would affect the adhesiveness. Applying to a warm body, but not sweaty, may help plus holding the applicator down for a minute or two before removing and then pressing gently with the flat of your hand for a few more minutes. After all that, you should be find, unless you knock it quite hard.
Good luck. I'm sure you will love having the Libre and you won't want to be without it after your trial