nanoDLP

Hi, new to forum and resin printing. Love the nanodlp on pi. Have had some good prints of makerook and tiny castle using iPad screen and photocentric daylight firm grey resin. Have hit some problems recently with early layers after the burn in layers. See attached photo. Looks like the layers are undercured then sort themselves out. Have checked z axis for wobble and backlash, all as tight as a Scotsman's wallet (I am Welsh, we have our foibles too). Anyone with similar experience or any ideas?