Diagnosis
Stereo pairing is software-managed, so when it glitches, it's usually a communication hiccup between the two speakers or with your device.
How to Fix It
Break the pair and remake it. In the Home app, find the stereo pair room, tap the three dots, tap 'Settings', then tap 'Ungroup Accessories'. This turns them back into two separate speakers. Then, with both in the same room in the Home app, tap and hold one, tap the gear icon, tap 'Create Stereo Pair', and follow the prompts.
If That Didn't Work
Ensure both HomePods are on the exact same version of software (check in Home app > Settings). Restart your iPhone or iPad. Also, make sure both HomePods are on the same Wi-Fi network—if one is struggling, it can cause sync issues.
Heads Up
For the best sync, use an Apple TV 4K as your home hub if you can. The tvOS hub manages stereo and surround sound timing more precisely than an iPad or HomePod acting as the hub.