Yes, the Atlas is one of my favorite sounding and flushing toilets. But if you continue to have this problem, either pinch the clip down or turn the black knob left , because I do not know what kind of Fluidmaster you have. It could be oversiphoning. If you do this and continue to have the problem then I would suggest a plumber to look down the drain.
If you do this and continue to have this problem, then allow me to suggest a Toto Drake.
Please note I am not a pro and I may be wrong.