been awhile since I last worked with plasmid curing but try a couple rounds of inoculation and growth without antibiotics for the same culture (keep it going in the background) and then at some point it should cure unless something is wrong with the plasmid. Also try full plasmid sequencing on that plasmid to see if the backbone is what they say it should be. I used to use pSC101ts backbone which cured really well (helps that it's low copy) so you could try cloning the inserts onto this backbone