You might want to take a look at the work of a lady called judy thompson
Thompson Language Center | Speak English quickly and confidently when you know the patterns (not the details) of how conversation works. or look for the book English is Stupid. She specialises in getting non native speakers to pronounce words correctly. As was suggested above, the method of clapping and placing hands apart is really great for placing emphasis on stretchy sounds - vowels and short sounds - consonants. She use and elastic band but I tend to use the clapping method alongside it.
You can write the word on the board, place a large emphasis on sounding out J-A-CK and then if you here the final 'a' ask them where it is in the word. You can split the class into 2 teams and ask them to challenge each other to pronounce simple words cleanly, without that final phoneme. those who get it right score a point and those that don't have to forfeit a turn.
Are you teaching each phoneme as purely as possible? Without the 'uh' at the end? E.g ck at the end of jack is pronounced from the back of the throat and is a very short sound it isn't a Cuh as is so often taught. It's really difficult for non native speakers to to differentiate between the 2, they just hear a final phoneme.
Apologies if this is teaching grandma to suck eggs
Good luck with it and happy teaching