I just noticed this...
I agree with the others. I also have 9month rescue x-breed and she's loves playing ball now but when I got her she was all shy and unsure. Work on building a bond, getting close, lots of reward and affection. Everything's so new to your little pup and overwhelming. Give it time till your pup feels secure and knows that youre it's forever buddy.
Then, introduce the ball. Maybe start playing on your own and once he/she sees you it will want to join in, then strat throwing and see if he/she will chase it, call the pup over to you while it's got the ball and offer him/her a yummy treat, make a fuss. It takes time, patience, repetition and reward especially with a rescue pooch but i think you'll get there especially since it's a young kelpie....if not then there's plenty of other games you can enjoy together. Good Luck!