If it's flat you have not used enough priming sugar, or not left it long enough to carb up. If you are serving it from the fridge you should use about 7g table sugar per litre to achieve about 2.6 vols CO2 which suits the style. (I use 1tsp is about 4.5g sugar)
If its the Euro lager your instructions will tell you to leave it 12 weeks or so before you drink it, which does seem to improve it.
If you have two more kit cans of lager, improvements could include using at least 50% light or extra light DME, the rest being dextrose or even 100% DME. And/or brew short to 20 litres. Also a dry hop or hop tea of say 25g of Saaz or Hallertau will give it a lift too.
Finally lager kits are really the poor relation of beer kits imo, so don't expect too much.