Help needed with X3 - Midi track meters sometimes stop working.

I just wondered if someone could help me out.  I'm using Sonar X3 and I have a problem with the Midi track meters/indicator.
When I am using a Midi track like for example, using my M-Audio Axiom Pro 49 to play a soft synth like Absynth 5, to begin with, everything is fine and when I play the keyboard, the red light at the top of the Midi track meter lights up and the meter shows the green bars to indicate incoming midi data but then after Sonar has been open for a while, the midi keyboard still plays the VST but when I press a key, the midi indicator no longer works.  Also, when I start playback, the meter starts working again.
 
Incidentally, this doesn't just happen with VSTs or when using my M-Audio Axiom Pro. It also happens with my Roland System 1 or my Novation Ultranova.
Anyone got any ideas???