Printer cartridges for Dell 1320c printer not lasting

Hi. We have a 1320c colour laser printer and always buy the high capacity cartidges which until recently have been excellent and have printed about 2000 pages before requiring changing however recently the last 2 black cartidges have printed less and less copies before registering as empty despite no change to the type of printing we are doing. The first decrease gave us about 650 copies and most recently we have only printed about 100 copies before receiving a printer toner low/cartridge needs changing soon message. We thought it might be a one off cartridge problem but this is obviously not the case. My colleague thought that perhaps it has something to do with a firmware update that we received recently. Has anyone else had this problem and if so what did you do to resolve it? Thanking you in advance. Monique