What difficulty did Alexander Fleming which caused him to stop working on penicillin?

Fleming had found penicillin in bread mold in the year 1928. He squeezed the mold 'juice' out of the bread mold, but it spoiled, even in the refrigerator before he could do anything with it. He was unable to prove that penicillin was an antibiotic, and finally gave up in the year 1934. Others later figured out how to isolate penicillin and make it into a medical form.