I would say move on and hope that over time with increased fluency you will see improvements in pronounciation.
I agree with mesmark that it sounds like it's time for you to move on. I think sometimes things just click in after you've left it mull it your head for a little while and started learning something else.
Also, with adults of the age group you mentioned, it might be really difficult if not impossible for them to reach the exactness in pronunciation that you're hoping for. Perhaps start the class with a 5 minute session focusing on one sound, or comparing two sounds [r] v. [l], for example and practice for accuracy for that time only. Mouth diagrams
help a lot to explain exactly how sounds are formed. If you have the time, making a cassette or CD for them to listen to at home might help.
Oh, one activity I really like that helps a lot with pronunciation accuracy is the simple dictation. Prepare one or two different sentences for each student, or a different list of words for each, and make each student
read their sentence aloud for the other students to write. Being forced to be speak so accurately because all of the classmates are listening so attentively might help your students remember both the word and the pronunciation.
Hope this helps.