That happens from time to time - its a problem that should be sorted once burnside finishes moving balance queries away from accessing bitcoind and onto reading from a database table.

Basically the system struggles when there's a lot of activity occurring - as it's taking way to long to carry out actions.

Another symptom you'll sometimes see is that when you load market it shows an out of date or empty values for number of shares held and/or available balance.

Both problems are solved by refreshing every few seconds until it works.