Several blogs have reported that United Airlines is currently unable to book award space on Singapore Airlines and Thai Airways. In fact it goes beyond that to include TAP Air Portugal as well.
They are able to book the space sometimes — but when space gets cancelled out by a partner they’ve got a real problem on their hands. It makes sense to stop making these bookings until they can get the problem fixed, recognizing this is a huge inconvenience to members (especially those trying to change close-in travel plans).