Birchy you should be able to cast a light fly (like a dry) easier and farther than a nymph rig or a large streamer. I may be making an assumption here but If you started fly fishing with nymphs and streamers you are probably used to the weight of these heavier flies helping load your rod. Also the timing for heavier flies is different than with very light ones which also may take time to adjust to.
Like others have said, make double sure you are using the right line weight and that you apply sufficient power forward after your line has straightening out behind you on your backward cast. Be careful not to apply too much power too fast on the forward cast. This will cause tailing loops which you don't want. The perfect forward cast starts out gradually but accelerates. Once all the line is moving forward, plenty of power can be applied.
This post of yours seemed quite strange to me as I learned the opposite way casting only dries then moved to streamers and then nymphs. Maybe that's why I have the opposite problem. I have trouble making very short casts with a 6 weight. I can do it, I just have to make a few to start getting them right.