There are several reasons that you might not get a good enough erection or it could take longer:
1) It is powered by two AA batteries. After a while, all batteries cease to provide as much power as they first did. Just put in new batteries.
2) You may not have a good seal around the base of your penis so the vacuum is not secure. Make sure that the rubber bushing is properly lubricated so no air leaks out where there is no lubricant to seal the area.
3) Sometimes your penis may be at a slight angle or otherwise off in some way so your erection is mediocre. If that happens, press the red release button on the top of the motor and release the vacuum. Your penis will return to a more flaccid state. Activate the motor again and it will usually proceed to enlarge normally.
4) While a sexually-stimulated penis is not necessary for the device to work, it sometimes helps. Before putting the tube on, stimulate your member until it responds. Your partially aroused penis will then be easier for the pump to enlarge in the penile tube.