Bed levelling or retraction.... most likely bed levelling given your description of 'too high for the circles'.
Spend some time levelling it. Those Mk2b heaters like to warp over time, or between heat cycles...
What is your levelling system? I use a feeler gauge rather then paper as it can't be squished under the nozzle and you can ensure it is always accurate. I use a 0.25mm feeler gauge and then set Z-offset from 0 to 0.05, depending on material.
On the other hand, it could be retraction. It looks like the start of your circles are thinner, which may mean you are losing steps in the extruder or you have not tuned retraction. losing steps in the extruder means that when you make a retaraction, the filament may not be returning to its original position, thus there is less pressure in the nozzle and a thinner line is made at the start of a new line. Perhaps this is why adhesion is poorer until the extrusion picks up a bit. Check your wiring, stepper voltage etc. Problems that come up over time are often wiring related.
My gut feeling is bed levelling, as even a slight bit of under extrusion should not be a huge problem for adhesion. In fact, having a bit more 'squish' on your first layer should solve adhesion if it is under extrusion.
Does this happen if you print everywhere on the bed? Does adhesion improve towards the edges of the bed? If so, its probably a warped bed and a levelling issue.
Edited 1 time(s). Last edit at 11/04/2017 07:20AM by Origamib.
