Wlan Adapter Monitor Mode Changes Too Fast

Hi friends,
i have a problem with wlan adapter: I use a kali on VirualBox. I can use (sometimes) immediately it in monintor mode. The adapter shows me the channels- it is perfect. But after 5-6 Min. the channels change too fast, i see nothing. When i want to list the channel again, I see just the numbers of channel (ch1, ch2). I have to reject the wlan adapter and then i have to bring it in monitor mode. Can anybody help me? why does it happen?
If it does not clear, pls write me.