Trouble cooking steak to the standard internal temperature
There's a few things that could be going on here:


cut: not all steak cooks at the same rate. Really tender cuts cook faster than some of the tougher, more flavorful cuts, increasing cooking time by up to 50%. Cuts like tenderloin, filet, and loin (US) - Sirloin (UK) are more tender and cook faster. Denser cuts like sirloin, top sirloin, and bottom sirloin (US) and rump (UK) cook slower. If you are cooking a denser cut then you simply need to cook it longer. 
Pan heat: Most chefs have really good stoves that produce load of heat, most mere mortals have average stoves which aren't as powerful. Medium heat on a professional, or very high quality stove is hotter than the medium heat on an average stove, so try cranking it up to full blast. Also, make sure your pan is fully up to temp. I cook my steaks on a cast iron skillet, and I let it heat up for 10 minutes before I start frying steak
Heat contact: even though steak is full of fat it takes some time for this to start working, so coating your steak with a bit of vegetable oil (not olive oil, it burns at high temperatures) will make sure it gets good heat contact


So my advice would be to get the pan hotter and cook it longer. Try adding one more minute per side. 

EDIT:
@kenny says that he is cooking loin, so assuming it's 3/4", or 2cm thick and looking for medium done-ness I would cook the first side for 5 minutes and then the second side 3 minutes. The uneven times are to make sure it cooks evenly. So you need a total of 8 minutes cooking time. I don't do the flip every minute method because the uneven times work for me, and it lets me do other prep.