Trebuchet - Why won't the darn thing shoot further

After much messing around with the trebuchet, I have extended the arm, shortened the sling, bent the pin and increased the weight and although it is throwing calmer, it just won't throw further. What the heck is going on? Clearly I am supposed to know the answers here, but I don't and I have no idea why not because it should all be so simple..... If you would like to help support the channel, visiting the stores helps, and signing ups for the news letters and subscribing is a great help.