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Possible explanations:
1. Tilt. If it is only on one side or corner of the image.
2. Out of focus.
3. Not thermally stabilised.
4. Wrong spacing for your flattener. Here is Roland Christen's advice on spacing a flattener:
Using Roland's method which states that if you move the focuser in and corner stars get better... you need more spacing and if you move the focuser out and corner stars get better you need to reduce your spacing.
Have you changed the spacing from when you had good stars?
5. Collimation is off, have you moved the scope before you noticed it or bumped into it etc?
Hard to tell from the small photo, a wider field shot with lots of stars would help.
So this same setup only just started doing this? Have you removed the scope and remounted it or banged it somehow?
I suppose a flattener could be banged or dropped and go out of collimation but have never seen that myself.
Greg.
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