It looks like under-extrusion. This can be a common issue with flexible filaments. You can up your feed/flow rates on your slicer which would help. The best solution would be to physically prevent the filament from going anywhere but the nozzle. This could be done with a piece of tubing between the extruder motor and the top of the nozzle.

I hope this helps.

Regards,
​​​​​​​www.3dexfilament.co.uk