Do not modify the gun, check your manual. I assume you have the manual?
You may have put the little plastic piece (forearm retainer) backwards. The gun works as shipped. I assume it worked before you disassembled it.
"Socialized Medicine is the Keystone to the Arch of the Socialist State.” -Vladimir Lenin
"I predict future happiness for Americans if they can prevent the government from wasting the labors of the people under the pretense of taking care of them." - Thomas Jefferson (An early warning to Obama care)