OK there is a technique to this and if it makes you feel better I also got it wrong initially. Make sure you are trying to re-attach the fore end the right way round not as stupid as it sounds, apply a slight twisting action which will help, however when the gun is very new a spray of light oil will work wonders. The procedure is easier if you are sitting down and you brace the butt end of the gun between you legs (God this is starting to sound like a porn film script), be careful once the fore end slips over the magazine it will collide with the action if you are using excessive force. The barrel can then be located and the three components locked together with the knurled nut.
I hope this helps.