sample track glitch

Hi,
95% of the time whilst working with the sampler track the waveform will disappear after ive worked on a different track. when i go back to a sampler track and click the sampler track open/close button to open up the editor the waveform is missing and i have the ‘drag sample here’ message showing. i have to click on a differnt lower zone tab lke editor or chord pads then back to sampler control tab for the waveform to re-appear for the current track i am working on.
anyone else have this issue. im on cubase pro 9, windows 10 pro, intel i7 7600, 32g ram.

thank you