Do you see the percentages before you fire??? If its red and low, chances of hit or penetration are low. Also make sure you have a clear line of site, I have forgotten to make sure to see the marker before firing and I thought it didn't fire, but instead it hit a tree or a rock
Also it depends where you are relative to the tank, if your 45 degree to the enemy tank's body, the chance of penetration unless your gun is huge and his armor thin or weak is little.
Make sure you shoot when you have a green light and a high percentage, I could guess one of these reasons is the culprit. Unless you guys are fighting in the EXACT same tanks, then I have no idea - just luck I guess