If your Chevy truck seatback doesn't lock, there are a few steps you can take to troubleshoot. First, check the latch mechanism and make sure it is properly connected and not broken. Additionally, inspect the springs beneath the seat and ensure they're in good condition; if they look worn out or broken, then replacing them may solve the issue. Finally, try lubricating any metal parts with WD-40 or a similar product to ensure smooth operation. If none of these solutions work, you may need to call a service technician for further repairs.