We often talk about a ball 'going flat'. So 'the ball went flat'. Perhaps 'the ball went flat after only a week' or 'the ball went flat immediately after I had pumped it'! We might use this term for our bicycle tires 'the tire was flat' or 'the tire went flat after a few miles'. A ball can 'burst' as well but that usually implies an explosion or at least a loud 'popping' sound which means the ball has been 'punctured'!

Q. What words describe a ball losing air? A. Commonly, we would say ‘to go flat’. The ball went flat over the winter. Where’s the pump? It wouldn’t be technically incorrect to use ‘deflate’ for a ball, but not that natural. We’d more likely say ‘deflate’ for smaller losses of air, or in a more technical setting. For example My air mattress does deflate a little overnight, but not enough to be uncomfortable. Note that ‘loses air’ and ‘has a leak’ are perfectly natural and native things to say too. ‘This ball loses air. Throw it out and buy a new one.’ ‘I think this inner tube has a leak. See if you can patch it.’

The ball deflated.

Not necessarily. That's why context matters; which few posting here seem to grasp.