Is this a breach of contract ?

[Karl27]

I have worked for a cleaning company for 6 months now and my manager has been making mistakes when doing the rota’s now and again. First it was not putting me in on one of my set days (Thursday/Friday/Saturday) I am contracted to 18hrs a week. I then raised the issue with him. He then replied with “I’ve informed my manager of the issue and it will be resolved”- I still to this day haven’t received a payment for the missing day after continuously asking. Secondly he put me in as working at another site even though I didn’t. Then yesterday 9/05/19 I checked my rota for this week he hadn’t put me in for that day so I rang him left a voicemail and a text. He finally rang me back today saying that he can’t make my hours upfront the missing day. He can’t change my hours without asking can he?
Thank you for any responses

[ULA]

What does your contract state about your hours of work and the days you work? Does the contact specifically say a set number of hours or "up to xxx" number of hours?

Where you still paid for the hours you worked even if it was not at the correct site or is there a dispute that you worked because the correct site was not given by your manager?