Late changes to mission design increased launch mass, requiring change in launch vehicle
Chandrayaan-2, originally scheduled for launch in December 2016 and most recently scheduled for October 2018, will now be launching no earlier than 3 January 2019, with its lander and rover touching down near the lunar south pole in February. The launch period extends to mid-March (TOI). ISRO chose this week to announce the naming of the lander "Vikram" in honor of the former ISRO chairman Vikram Sarabhai (HT).
As with most ISRO mission news, there isn't anything about this on ISRO's websites. In fact, their website still lists the Chandrayaan-2 launch as happening in "second half of 2018." I have to composite a story from reports on newspaper websites. Therefore, what follows is second-hand information, and is unverified. I'm indicating newspaper sources in parentheses.
According to M Annadurai, director of the U R Rao Satellite Centre, the orbiter is ready for launch, but the lander and rover are still being tested (NDTV). At least one source says that "a further delay may not be a surprise" (The Week).
Further delay seems likely. The list of late changes to the design of the lander is very long (TOI). A comprehensive technical review was conducted in June (or perhaps March; different articles mention different dates. Maybe both). The review board evidently found the lander mission unacceptably risky, and suggested a fundamental change to the mission plan. Before, the lander would have landed directly after separation from the orbiter. In the new plan, the lander will orbit the Moon on its own, checking out systems, before deorbiting and landing.
It sounds less risky, but is it? In order to orbit and then deorbit, the lander has to also be a fully functional spacecraft. The change in mission design requires the addition of a fifth 800-newton landing rocket at the center of the lander, as well as extra propellant in two new tanks, and a third new tank for extra pressurant. The mission has to add four reaction wheels and star sensors for three-axis control, plus their support electronics and software. These changes are in addition to others made to increase the chance of lander mission success: a change in the lander leg configuration to increase stability, and addition of redundant electronic components.
These late changes increased the mass of all the payload components (TOI). The lander's dry mass increased by 100 kilograms, to 1,350. The rover mass increased by 5 kilograms, to 25. The additional propellant and other changes resulted in a net mass increase of 600 kilograms, from 3,250 to 3,850.
The extra launch mass would have strained the capabilities of the planned launch vehicle, a GSLV Mark II. India considered using an upgraded version of the rocket, but decided that making the precious Moon lander be the first flight of the Mark II upgrades was too risky. A less risky course was to swap the planned rocket for a more capable GSLV Mark III. That being said, there have only been two flights of the Mark III to date (The Week).
Even with the additional thruster, there may be some issues with the lander design not performing well in tests (The Week). This Times of India article suggests disagreement between the mission team and oversight board on whether all these late changes to the mission design will actually make the landing less risky.