Depending on how conscientious your SM is the all money might be sitting in a current a/c earning 0% interest and that is why no income is shown.
A good SM will put most of the Sinking fund into a term dep for say 12 months, but the EC or OC can instruct the SM what to do with the funds (term and amount) at the AGM and nominate someone to oversee this throughout the year.
Bear in mind any income you receive is taxed at 30% (company tax rate) and so IMHO it is better to keep the sinking fund low and raise special levies when required.