Anyone who accepts credit cards is assigned a 'category' (called an MCC code). All of Eleyo's payment processing is assigned a code for Child Care. If a transaction is declined, it is not the Eleyo system that is declining it. A decline also does not reflect that the Eleyo system can or can not accept FSA. A decline reflects what the provider is allowing or not allowing.
In some cases, parents have been told that Eleyo is not an approved vendor. One possible explanation is the FSA provider only allows a narrow group of codes that it considers 'approved'. There is no option for Eleyo to change how the payment processing is coded.
Another reason for a decline is that some FSAs are set up when the user has to accrue funds to then be applied to child care (i.e. on a weekly or bi-weekly basis). If a particular transaction is denied, it could be due to lack of funds to apply specifically to the child care portion of the FSA.
Some parents report success contacting the provider and explaining the situation (i.e. that the charges are for child care). They may be granted an exception by the provider to allow the transactions to be approved. Every provider is different so it is up to the customer to discuss it with their provider and see if they will allow these transactions to be approved.