Mempool not accepting transactions: 'reject-reason': 'insufficient fee'

I'm using Bitcoin Core, and for some reason, it doesn't want to accept transactions when I already have other transactions in the mempool, even if those transactions are using different UTxOs and they have different non-witness data. I have enough to pay for the transaction and fees. I also make sure that the fees are greater than the vsize which means that the problem is not with the fee.

Can someone provide an insight about this specific reject-reason?

By default Bitcoin Core’s mempool data structure is limited to using no more than 300 MiB of RAM. If your node has been consistently online, it would currently be full of transactions that pay about 4.6 s/vB or more. It seems to me that the transaction you were trying to send paid less than your own mempool’s minimum feerate and thus was rejected with the reason "insufficient fee".