Last round? I'm betting mag spring. Possible it is a fouled mag tube. Must be disassembled and thoroughly cleaned. While it is apart, give the spring a stretch. If it is the spring this will TEMPORARILY fix it. The spring will relax again to the previous condition and the problem will resurface.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball