Gary Ross directed the first Hunger Games film to nearly universal acclaim and a record-breaking box office, but events transpired to prevent him from directing the second film in the franchise, Catching Fire.

The poster for 'The Hunger Games: Catching Fire'
There were a lot of different reports after the first film came out about whether Ross would be returning to direct Catching Fire. Most fans assumed that he would be back as he had admitted in interviews that he already has plans for the second film. However, due to time constraints and several other [mostly unknown] reasons, Ross ended up not signing onto the second film.
There was a major search for a replacement director, and eventually Francis Lawrence came on to replace Ross for Catching Fire. Lionsgate later announced that Lawrence would be directing Mockingjay Part 1 and 2, especially as everyone involved in the film was very happy with him.

Francis Lawrence is directing the remaining Hunger Games films
However, a new report now states that Gary Ross could be returning as producer on Mockingjay Part 1. According to the Hunger Games fan site Quarter Quell, the industry magazine Production Weekly listed Ross as a producer for Mockingjay Part 1. While this could just be outdated information, there is a chance that it could be legitimate - Ross has always been very passionate about the franchise, and if truly was scheduling that prevented him from returning to Catching Fire, then he could easily return for Mockingjay.
Gary Ross interview for 'The Hunger Games'