Nippon Life Benefits review: my own money Contributedto my FSA account cant be reimbursed for dental treatments to my daughter

I contributed my own money to a flex spending account during the time I was actively working for a company. I left the company, left the fund in that FSA account too. I thought I authorized the ortho. office to charge me on that FSA account, they didnt charge. The fund was still in my FSA account. By the time I contacted the FSA company, they denied to reimburse me for the payments I made to the ortho. office. They said the claim was beyond the window time they allowed. They still keep my $540 and refused to reimburse me.

Company: Nippon Life Benefits
My contribution time: 5/1/2022 - 8/31/2022
Please help me with this matter.
Thanks,
Jackie