I have no specific knowledge, I just follow the news on it.
My read is that the issues are relatively minor but tricky to fix - especially without alerting the media and triggering a slew of 'PUBLIC TRANSPORT BUNGLE' headlines.
E.g. some of the track is a little too steep to meet the maximum braking distance requirements, and now they need to have a back-and-forth with the manufacturer to see whether they can retrofit better braking systems or something. DISCLAIMER: This is just a thought-bubble, I'm not a train expert! I'm just thinking the issue(s) seem to not be visible/obvious, so some kind of technical metric not being met, and a long delay to organise a technical solution with a third party would fit the bill.