XBT0* queues piling up in SMQ2

I have a large ammount of messages in XBT0* queues getting piling up in SMQ2

XBTO2___0006 797

XBTO2___0007 808

XBTO2___0008 852

XBTO2___0009 1.036

XBTO3___0000 1.098

XBTO3___0001 945

they are stopped being processed too - what should we do to have normal message processing , why do we have such behaviour with queues?