I suggest to change first the eprom eraser for future use 
Chinese eprom eraser can be dangerous for eyes and don't have security ! (be carefull for your eyes !)
I personnaly own this one below which works fine, but minimum erase time is 30min (prefer to perform 2x 30min on the eprom before using them) and sometimes more, not less.
Less time = more erasing failure rare. Do not put to much eprom in eraser, eraser light must be in front off the eprom glass.
old chips have sometime higher failure rate
https://gie-tec.de/product/tuv-exposure-boxes/?lang=en
in 4W or 35W UV LAMP model

Chinese eprom eraser can be dangerous for eyes and don't have security ! (be carefull for your eyes !)
I personnaly own this one below which works fine, but minimum erase time is 30min (prefer to perform 2x 30min on the eprom before using them) and sometimes more, not less.
Less time = more erasing failure rare. Do not put to much eprom in eraser, eraser light must be in front off the eprom glass.
old chips have sometime higher failure rate

https://gie-tec.de/product/tuv-exposure-boxes/?lang=en
in 4W or 35W UV LAMP model
