This problem can have multiple reasons. If your device is using Android OS, you should know that in order to make a video call on Skype, your OS should be at least version 2.2 and you should have a minimum of 27 Mb free. Also, if your phone has an ARMv6 processors (like Samsung Galaxy Ace, HTC Wildfire models), you can use Skype but you will not be able to make video calls. Also, if your phone has an Intel processor (like Motorola RAZR i, Orange Santa Clara models) Skype isn't supported. For more information regarding this matter, check the following official website.