The extractor spring insert is elastomer*, not hard plastic, so it should not have cracked. If your insert has cracked, or was plastic, it and possible the spring where junk from the beginning.

The very short version/response,
Buy another extractor spring, and this time; order a Colt M-4 black insert extractor spring to guaranty that the one you receive is an in-spec part (insert correct and the spring to the correct tension).

*
Synthetic rubbers
· Butadiene rubber (BR)
· Butyl rubber (IIR)
· Chlorosulfonated polyethylene (CSM)
· Epichiorohydrin rubber(ECH, ECO)
· Ethylene propylene diene monomer (EPDM)
· Ethylene propylene rubber (EPR)
· Fluoroelastomers (FKM)
· Nitrile rubber (NBR)
· Perfluoroelastomer (FFKM)
· Polyacrylate rubber(ACM)
· Polychloroprene (CR)
· Polyisoprene (IR)
· Polysulfide rubber (PSR)
· Semi-Conductive Fluorocarbon with nano carbon tubes.
· Silicone rubber (SiR)
· Styrene butadiene rubber (SBR)

Thermoplastics:
· Thermoplastic elastomer (TPE) styrenics
· Thermoplastic polyolefin (TPO) LDPE, HDPE, LLDPE, ULDPE
· Thermoplastic polyurethane (TPU) polyether, polyester
· Thermoplastic etheresterelastomers (TEEEs) copolyesters
· Thermoplastic polyamide (PEBA) Polyamides
· Melt Processible Rubber (MPR)
· Thermoplastic Vulcanizate (TPV)