Didn't we just have a many-page thread on this very topic?
I recall that, setting the Rigol scope's memory depth to maximum, the capture mode to 'Normal', and then doing a single shot capture filled the available memory with data that was decoded as you scrolled left and right.
The 'issue' is that, by default, the memory buffer is auto-sized to only capture what's on the screen.
The OP should try playing with the settings I just mentioned. I believe it's a feature, not a bug.