volume issue on ipad 2

Hi. My son has an ipad 2 with a broken volume button. After seeking assistance here, I have set it up to control the volume through assistive touch.

Today it is playing up. Im not sure if it is related to this or something else. The assistive touch says the volume is at its highest. I can play music and hear it. Apps like talking tom, talking angela play and I can hear them. But every other single app I cannot hear. Im at a bit of a loss. Any advice?