what brand of pla are you using and what temps and speeds are you using ?

Got to admit pla layer adhesion is not something i've ever really had issues with.

Under extrusion would be the only thing that comes to mind.

what slicer are you using ?
And what is your extrusion ratio and extruded bead size set to ?

The extruded size is NOT the same as the nozzle size.
And - in my experience - slicers have a tendency to massively overestimate the extruded width.
On a 0.4 mm nozzle most slicers want to set the extrusion width to around 0.5.
I've measuired my extrusions widths and use those settings.
My delta with the 0.5mm nozzle runs at 0.52 and the other printers with 0.4mm nozzles are both currently set at 0.4

So that's worth checking.

If you don't yet have aet of digital calipers - get one :-)

Abs expandsa lot more than pla - so the extrusion width is always going to be larger, which might account for why abs works better than pla.