Monitor missing Peak, then falsely reporting Peak

I've been using my Clearblue touchscreen monitor for a couple of years now, and have always had very predictable cycles. Starting 6 months postpartum, I've been using it again but find the monitor's accuracy is severely compromised. For 3 out of 4 cycles now, it's missed my LH surges (confirmed by an LH/OPK test and subsequent cycle markers). This cycle, it reported an LH surge on Day 9 of my cycle despite a negative reading on the OPK. I kept testing with the OPK, and today I got a positive with that but a Low on the Clearblue monitor. Any idea on why the monitor has become so unreliable?

Other details: OPK is from Wondfo, LH detection threshold of 25 mIU/ml. Clearblue monitor item model number is 633472600227, LH detection threshold of 40 mIU/ml; batteries are new; monitor reset after each postpartum cycle.