Has a customer shown you they have money in their account, but you're still getting an insufficient-funds error?
The only way to know for sure is for the cardholder to contact their bank about that specific charge. Banks make funds available differently. The money may have been pending rather than available when the charge ran. Stripe and the app only see the decline code the bank returns; the bank has the real reason.
For everyday declines in general, passing along more card details can help. The more a bank has to verify against, the more likely a charge goes through. When you can, collect the cardholder's name and ZIP code along with the card number.