I have replaced both Slick mags and spark plugs during my recent condition inspection. The mags are timed to engine at 25 deg BTDC as indicated on the engine data plate, same as previous timing. The engine is IO-360-M1B, I could do LOP before CI. Engine runs fine ROP (I typically setting for short local flights) after the CI. But in last flight when I tried to do LOP, EGT on all cylinders just wouldn't drop when leaning. The EGTs kept going up and up to 1600Fs, and engine started running really rough, but the EGT will not drop. I know the engine is definitely running LOP based on fuel flow rate, but why I didn't see EGT drop. The leaning was done between 45%-55% power setting. What can cause this? I had the injectors cleaned during CI. I will double check the timing, which I already did before putting the cowls back. The mag checks are fine, both ground and in flight.