Gharromish71
Strange, just 2 days ago I was able to top up mine (which was also in an expired state for a couple of days) on sbb.ch with that button under the greyed out balance details. Topping up is not supported on the swisspass.ch site, only on sbb.ch.
Now before going to a POS, maybe try a different browser first, just to exclude some cookie or browser cache issue.