Experienced this myself in my most recent VO2Max block - general consensus was carrying fatigue as the block progressed was limiting ability to raise HR.
Depending on school of thought you are following, whilst striving for >90%MaxHR is great, working at 88% is still going to provide some adaptation one would hope and if you are hitting power targets then I would consider the VO2 work to be successful in the round.
This thread may be of particular interest and I also link my post where I raised a similar query