It is not very common to unveil secrets of Chinese space program by people involved directly with it. Especially when maiden flight of largest rocket ever created by Chinese industry is concerned.
Few Chinese specialists present during launch of Long March 5 on November 3, 2016, from Wenchang Satellite Launch Center announced on Chinese social media unknown details about this mission. It seems that Chinese information policy regarding space flights has changed - few years ago probably something like that would not be possible. Thanks to this change, we know how dramatic events preceded launch of Long March 5 - largest Chinese rocket ever created.
Four days before flight appeared first problem. On October 30, 2016 two days after rocket was rolled out from assembling facility to LC-1 launch site it was detected that launch table is misaligned. Launch table is the part which is reflecting whole thrust generated by engines. If it is not installed with specified angle, nozzles of rocket engines will automatically adjusted by rocket's onboard thrust control computer even up to maximum values. This can destroy part of the launch tower and even cause disaster of rocket. Of course it is possible to reduce this problem by disassembling boosters and adjusting them in the factory but it means months of delay. CNSA decided to take a risk and luckily, this time nothing wrong happened. But it was not only issue with first flight of Long March 5. Later, as we remember, launch suffered for additional delay and was postponed for over 60 minutes. It was result of additional problems with leaking LOX from two from four boosters supporting Long March 5 during start. It turned out, that there is no leakage and probably failed sensor or data were wrongly interpreted. Next issue appeared at T-90'. Core of the rocket, equipped with two YF-77 engines seemed to have problem with fueling system - turbo pumps, which should be chilled with fuel, reported to keep too high temperature. Launch was postponed again for 100 minutes. Specialists decided to input commands manually to valves - it helped and about 70 minutes from launch (which took place finally at 12:43 GMT) chilling procedure was resumed. Another issue was recognized after switching rocket to battery power, but it was quickly eliminated by engineers in control center. After launch and smooth flight to orbit, when problems appeared to finish, second stage of the rocket started its propulsion to place Yuanzheng-2 upper stage on GTO orbit. According to some Chinese space forums, propulsion was working not as long as it was planned and was cut off 11 seconds earlier. It was not officially confirmed, but it is worth to remind, that problems with countdown were also not mentioned during live broadcast. Due the shorter time of burn of second stage, Yuanzheng-2 performed prolonged first and second burns to compensate it.