Was flying towards Gatwick today, level at FL80, 220 IAS, LNAV and ALT HOLD and received ATC instruction to descend to 4000'. I dialled in 4000 and selected LVL CHG but was surprised when descent rate held pretty steady at only -250fps. I eventually had to use VS to increase rate of descent and by the time I had leveled-off at 4000, I had missed my next change of heading. I'm puzzled now as to whether I suffered a system glitch or mis-operated the descent due to mis-understanding procedures. I believe that LVL CHG is appropriate for a descent of 4000, if neededing only a few hundred feet change, I'd use VS. Earlier in the flight I had experienced a 'BUFFET Alert' and speed dropped from 267 to around 210 in the early descent under VNAV. I put this down to a glitch caused by weather (using ASE), but it has happened on several flights and I've no idea why. Suggestions much appreciated.

Thrust setting? Speed? Screen shot might help.

I wouldn't be surprised if your throttle was not set to idle. * Orest

The aircraft is controlling to pitch to maintain airspeed in the descent (in this case, at 220kts). Did throttle go to idle? I.e., what was list in the FMA?Trevor

Auto-throttle . . . . . . . . Engage.