For me the picture you linked is under extrusion.
So posibly :
- partialy obstructed nozel (check by mesuring the diameter of the extruded filament)
- extrudeur default (slip of the filament …)
- flow too low ( slicer setting )
- printing temperature too low for the used filament ( slicer setting or defective temperature sensor …)
- bad quality filament reel , the filament diameter lower than 1.75mm or not constant
- …
But on a delta … you have to also check the head movements … the rod free sliding, …
Hoping you will find the solution, have a nice day !
Edit : it was due to a bad cable protection causing short cut (creating false temp. drop under 170°C stopping the extrusion) of the temp. sensor cables … as i get it from an other post of human … see : Temperature jumps up and down