Printer will print perfect calibration cube but spaghetti for everything else..

[3DPHRINT]

Hi - New to this and scrabbling about for some pointers. I have an Ender 3 Pro. After setting it all up I found a 20mm calibration cube, then I did the slicing bit on Ultimaker Cura - Used PLA that came with it. It came out perfect and was 20mm all round. Printer heated base reasonably flat. After a couple of days I thought I'd try and print something useful to me, got it off Thingiverse and this time it would not stick to the base and nozzle was dragging it around - So I double cleaned it with alcohol and after much messing about with slicer, clicking adhere tab, quality changes etc decided to change reel to PETG (with corresponding new slice file/temps). Still no good - I changed the nozzle (hot little buggers are they not...) still no good. Last thing I just tried was putting PLA back in and did a calibration cube - PERFECT. Any ideas please - is it possible it is a bad print file from Thingiverse?. Any assistance appreciated...