If you are building the bootable USB stick on a 32-bit machine it cannot make a 64-bit bootable OS by default because it does not have the 64-bit Bootsec. So first clarify, do you have the 32-bit W7 ISO or the 64-bit W7 ISO? And if you are using the 64-bit ISO then, are you trying to create the bootable flash drive on a 32-bit machine?
If you need the 64-bit bootsec.exe you will have to try and find it from the Microsoft website. I have 64-bit computers so I have never needed to get it separately myself.