I use the other portal, which some refer to as the "old" e-gov DHS website, could be the reason?
Did you guys have also pending i751? I got a text earlier saying that my i751 was approved on April 23rd, which was my interview day, yesterday got the oath notice dated 26 and was wondering why there was no update on the i751 but apparently it was just a delayed notice (received 27th dated 23rd)
as for ceremonies the IO told me that the next wave will be in June, people taking their Oath in May were notified in April, so maybe we will get scheduled for June and notified in May.
if you have an approved i751 and you're just waiting for the oath, things are good, do not over think about it, eventually the appointments will be sent out...