My Iphone 5 screen stops responding properly when plugged in with case on

Whenever I have a case on my phone, while on the charger the touch screen will not respond. Lock & home button still work. I have several different plastic/gel cases and they all have the same problem with any charger. I never have problems with my screen except for the unique situation when I have a case on and it's also plugged in. If I take my case off, it works fine while plugged in, and vice versa with the charger. The plastic doesn't come close to touching the screen so I don't know why it would effect the screen. It's really frustrating and I know an easy fix would be "take your case off when charging!" but I shouldn't have to do that, and of course accidents happen so I like to have my case on at all times. Please someone give me an idea to why this is happening to my iPhone 5. Most recent software on the device, and it's an 8Gb phone