Poor per tile quality on top surface

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Dear All,
We've been sequencing CRISPR guide RNA libraries for a while and had several high quality libraries sequenced on HiSeq4000. However the latest one shows this QC fail with very poor sequence across almost all of the top surface tiles. We repeated the sequencing on another flow cell and saw exactly the same result.

We add a custom sequencing primer, but have used the exact same library construction and sequencing protocol multiple times before.

I would be extremely grateful if anyone has any helpful suggestions what could be causing this?

Picture attached.

[GenoMax]

Have you consulted Illumina tech support? You are not doing something totally custom that is unsupported by Illumina?

[misterc]

You likely lost focus of the top surface. Check the FWHM on top vs. bottom surface for the run. I know some have had intermittent issues with losing focus on this platform when there's a read (even prior) that has zero base diversity. HiSeq 4000 doesn't always recover from this.