MK3S Filament Adhesion Issue (Under Extrusion?)
The machine: MK3 that has been upgraded to MK3s, plus this hot end kit: https://www.matterhackers.com/store/l/e3d-v6-gold-hotend-full-kit-175-24v/sk/MQ36W84W
The issue: Printing with PLA, tried with several brands that work fine on my MK3s. All prints look fine, but they have almost no adhesion between layers both Z and X/Y. Consequently, the parts come apart very easily. The photo shows an example (pushed open to show how easily it breaks open). 
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It seems like an under extrusion issue, as everything prints 'correctly' it's just not sticking. I've tried tinkering with the flow rate and that gets it to stick but the prints don't come out very clean. I've never had to mess with the presets much, so I'm tentative about doing more harm than good.
What's the best method to determine what the problem is?
RE: MK3S Filament Adhesion Issue (Under Extrusion?)
Post some pictures.
Did you reset the PINDA and run calibration when you changed the hot end?