L322 gearbox revving high but not engaging

Hi, fairly new to this forum. Just wanting to know if anyone has had the same problem as me and if they have any clues to what is going on. My Range Rover tdv6 3.0 l322 was really struggling to pull up a small bank today and when stopping at traffic lights then setting away again it took forever to start to move and didn't pick up the pace until about 30mph. I have recently had gearbox oil and filter changed and the car itself has only covered 109k miles. I am wondering if anyone knows as to why this problem has suddenly come on. Sometimes if I turn car off then on again it will engage into gear then suddenly silps back out and just revs without moving... any help would be appreciated.. thanks in advance