The back case has a long peg that can snap if it's too weak. Consider printing it using one of these opt...ions:
High perimeter count, so the peg is nearly solid
High infill value, so the peg is nearly solid
Use the "Bolt" version of the case, so this important part is metal
Optimize your print settings for layer adhesion - this is usually a higher nozzle temperature
Print spring with 100% infill. Ideally, the filament path should run the length of the spring. This can be achieved by forcing the infill to be horizontal instead of diagonal or by setting the perimeter count to a high value.