Rigol DS1054 Trigger problem (sometimes it wont trig random)?

[fl]

Hi

(If wrong posting sorry, please move it)

Problem is the trigger system on my DS1054, sometimes it wont trig random?

Problem is when signal is small compared to the scale setting.
Can be a 5v signal, with scale set to show a small signal, ex set scale to 5v no trig, when change to 2v it trig.
Off cause i set the trigger level to trig the signal...

I have other scope and no problem on them.

Suggestion?