2700 Workshorse gearbox still not been fix

I took my Kia Workhorse for the first service and told the workshop manager about the gearbox problem. He did not fix it after my third time for the Kia service and on 20 000km I complain for the third time about the gearbox. This is still under guarantee. I took the new workshop manager in the Kia workhorse to show him the problem, he said he will do something but we still not have any response from them. They make promises they will phone back but they do not so. My bakkie was for a week there but the problem is not been fixed. They say the guarantee is been approved to settle the gearbox problem and when I went there this is not been fix yet. They promise me to settle the problem and phone me and they still not do that. What must I do, is there anyone who can give me good service and fix my problem about the gearbox?