40D issue

Last days I noticed that my Canon 40D may have an issue. While taking a simple shot: it confirms focus/beeps, (I re-compose or not) then full button press...but doesn't fire or fires after a long delay (while holding full button press). As if the full press does not make proper contact.

It's very frustrating missing the shots...can somebody tell me what's going on here? And how do I fix it?

It is set in one shot focus, center point, burst fire mode, Av mode, lens 50mm 1.4 in AF mode. The shutter count about 20k, not abused, no dust, no smoke, looks like new...