My interpretation is that the 3/8" test can only be used if there was a lapse in the period of effectiveness, i.e., six months. If the welder then fails this test, as indicated in 4.24.1.2, a "complete" retest can occur after further training and practice, using the 1" plate to retest and qualify for unlimited thickness. I agree with you that it's only one attempt. If a welder, who previously qualified with a 1" plate can't full pen a 3/8" thickness, then he/she needs to requalify with 1".