If your JB has been reduced, and you haven't called to the CWO to advise them of your change in circumstances, that may explain why your rent supplement has been stopped. The CWO will have seen on their computer system that you're now working (and that your JB has reduced), but won't have the details of your income - which he/she needs to know to calculate your entitlement to Rent Supplement.
It's not the hours per se that affect your entitlement (although you won't get Rent Supplement at all if you work more than 29 hrs per week), it's the earnings, as SWA is a means-tested scheme.
As I said in my earlier post, bring payslips to the CWO as soon as possible so that your supplement can be reviewed.