Israeli media is reporting that Hamas is complaining about three things (N12, Times of Israel, Ynet):
That not enough aid was transferred to Gaza City. Per Israeli reports, 50 of the 200 trucks that entered today were supposed to go to the northern part of Gaza.
That the prisoners are not being released in the specific order that Hamas wanted them to be released (apparently they wanted it so the oldest are released first - although this may be by who had been in prison the longest)
That the IDF is not allowing Gazans to return to the northern part of the Strip (reportedly, this was actually an explicit part of the deal).
Israeli media are also reporting that a Qatari delegation have arrived in Israel to help facilitate quicker communication with Hamas, and that Qatar has told Hamas to “stop playing games”, and that Egypt is also demanding they begin releasing hostages within the next hour. Israel has publicly announced they will resume the military campaign at midnight if the hostages haven’t been released.
Al Jazeera for their part are reporting basically the same, with some analysis that Hamas are “trying to reach a parity with Israel”.
UPDATE while writing this: while sourcing all these links and translating stuff, apparently the whole thing is now solved and the hostages are expected to be released in the next hour - 13 Israeli citizens and 7 foreigners.