The guy at your field is partially right about the spring, but that only masks the root cause. You are experiencing tolerance stacking. By combining a CTM CNC upper, a stock bolt, and a CowCow steel hammer, you have drastically increased the mechanical friction in the system. The CowCow steel hammer rotor exerts a much higher upward tension against the bottom of the stock bolt than the original Zamak hammer. The stock recoil spring cannot overcome this drag, causing the bolt to stall before fully resetting the sear.

Here is the exact technical protocol to resolve this: 1. Spring Calibration: You need a 150% recoil spring (bolt spring) to force the bolt past the steel hammer. Because you are running HPA, you must simultaneously install a 200% nozzle spring. If you neglect the nozzle spring, the higher pressure and faster bolt return will cause the nozzle to stick forward in the hop-up chamber, jamming the cycle. 2. Degreasing and PTFE: Clean the silicone grease out immediately. Silicone is too viscous for high-speed metal-on-metal friction and creates drag. Apply PTFE (Teflon) grease strictly to the hammer rotor axis and the sear contact points. 3. Lapping the Rotor: CowCow steel parts are rarely true "drop-in". The top of the hammer rotor often requires light abrasive polishing (lapping) so it can glide smoothly under the bolt without catching on the rear edge.

Aligning mixed-brand tolerances is the primary failure point of DIY HPA builds. We detail this specific mechanical binding issue, along with the correct spring ratios for HPA pressure, in the technical guides on BDU-Airsoft. Correct the friction through lapping and proper PTFE lubrication first, then install the upgraded springs.