PROLINE H7516R TABLETS PURCHASED

In November 2015 I purchased 3 x Proline H7516R tablets through our company from Pinnacle Africa. These were Christmas gifts for my kids which were only given to them on Christmas morning. The tablets have hardly been used and started giving problems. I returned to work in January 2016 and made contact with Pinnacle explaining the issues I was experiencing with these tablets, and to get a refund as the tablets are still brand new or alternatively replacements of a different kind, and the difference to be paid. I was asked to have it returned for checking, but in the interim was told that no refunds/exchange can be given as the units have been opened. Surely the units had to be opened in order for it to be tested, otherwise I would not have picked up these issues. Please advise a way forward to resolve this issue as I do not want these Proline tablets, but will rather prefer to get a refund and purchase something better. To be quite frank, if the units are still under warranty, surely these can be taken back and refunded or exchanged for better units, but unfortunately this is not the case. I want my money back as I am very disappointed in these Proline tablets.