Spectacles transition layer coming off

I have been using same optometrist for the almost 6 years at lephalale mall but my last pair of spectacles which i paid around R5900 due to transitions added to lenses could not make it to a 2 year period without any issues. The lenses transition layer came off whereby you can scratch them off with a nail, I have a pair of specs going for 4 years without any issues, I took glasses back for examination & they claim is a customer negligence, how can I damage glasses when I need them daily & paid a hefty price for them, the transition layer coming off is in the inside, normal wear & scratches usually occur on the outside which can be regarded as negligence, the optometrist has offerd to pay 50% for the lenses which i`m not happy of, I paid fully for the specs not disappoint not to come back later & pay additional money to fix some layer which is coming off. I am due for renewal soon I do not think I will go to the same optometrist again for a pair of glasses.