You probably got this figured out but, to cancel an order, click "List my active orders", under "My Trade History", and you can cancel there.
I too noticed that orders outside the ones currently listed, aren't showing up in the order table.
No, that is what is broken: it does not even show any of my sell orders in the list of active orders, just a few of my buy orders.
Basically it looks like it maybe only shows a few of your orders, maybe just the last few that you placed, whereas of course typically the ones you need to cancel tend to be the oldest ones.
For example I need to cancel my highest highball sell offer to place a lower low sell offer.
But none of my sell offers are shown on the active orders list at all, only buy offers, because after I had placed all my original sell orders starting way way up high highball offer and stepping down down down down in steps all the way to near where the sells meet the buys, I then went through the buy side placing buy orders all the way from outrageously lowball up and up and up in steps to near where buys and sells meet.
Only those last few orders, the last few buy orders as it happens, are shown in the active orders list.
Meanwhile although a bunch of the lowest price sell orders did get accepted by buyers, the active trading price-range has moved back down somewhat so I now need to move some highball sell offers down toward the active zone but none of them are shown on the active orders list so none of them can currently be cancelled.
-MarkM-