layer shift on newly assembled printer

Mega frustrating layer shifts on newly built printer. Green piece was the first one. No shifts. But it was the only piece printed at once. Then i tightened back belts. Also printed two other pieces at once, that were good. Then the blue pieces overnight. Layer shift on one axis. Now the purple pieces also shifted both axis. Are my belts too tight? Or is my spool tugging on the head. Blue was small 1kg spool. purple one is 2.3kg when it was new.