Last 4 batches have been pretty flat???

Hi all

I have been making kit beers the last year or so, the first half dozen have been pretty good. But recently my last 4 batches i have made have come out not very carbonated and lacking head. I dont know what I have done wrong as i havent really changed anything. the first two which were flat i had changed from using dextrose or brew enhancers to spray malt which i thought might have caused it. So the next two i made i just kept it simple but they are still pretty flat! I prime using 1 heaped teaspoon in 750ml bottle and left for 2 weeks before the fridge. One batch i even removed from the fridge for another week to see if it would carbonate more. Any ideas why all of a sudden they are doing this? Its hardly drinkable to me like this. It tastes fine though so i doubt there is an infection in all those brews.