Here's the thing: In a climb, when I have the MCP altitude set lower then the VNAV target altitude then the aircraft will hold its altitude as commanded by the MCP.The VNAV button is still lighted green and therefore, when increasing the MCP altitude one should expect that the aircraft will automatically climb.But I will always have to push the VNAV button twice in order to 'wake it up' for it to do so. Similar in a descent, When interrupting a VNAV descent by pressing the ALT HOLD button the aircraft will maintain that altitude.When pressing VNAV again, i'd expect the aircraft to continue its descent, but this doesnot always happen.perhaps it depends on the lateral mode being used at the time (LNAV or HDG select) as well, i don't know Is this behavior true to the real aircraft?

If the VNAV has to stop at an intermediate level (i.e., you have restricted the climb/descent by the MCP altitude), it will switch into VNAV ALT. Once in VNAV ALT, the only way to get out of it, is to either disengage VNAV and then re-engage it (like you have done), or you can simply hit "ALT INTV", which is similar to pressing on the altitude selector knob in the B747-400 and B777. Cheers DZ

If the VNAV has to stop at an intermediate level (i.e., you have restricted the climb/descent by the MCP altitude), it will switch into VNAV ALT. Once in VNAV ALT, the only way to get out of it, is to either disengage VNAV and then re-engage it (like you have done), or you can simply hit "ALT INTV", which is similar to pressing on the altitude selector knob in the B747-400 and B777. Cheers DZ

OK thanks, so it's all like it's supposed to be then, very good. I'll give the ALT intervention button a try next time and i will keep an eye on the PFD to see if VNAV changes to VNAV ATL and back to VNAV again etc..,