There are only 2 things I can think of. First put your new bow on a scale to be sure that the draw weight is really set at 50 lbs. It may say 50lbs on it but may actually be cranked down much farther. It sounds odd that you can go from shooting a 45lb draw with no problem to not even being able to pull back 50lbs which is only an increase of 5. That's why I think the draw may be set higher thanyou think. If this is not the case and both bows are at the correct drae weight then you will just have to strengthen those muscles that you use to draw the bow. If you look around you will find strength training devices just for bow hunters. Good luck.