You have to understand the difference between tax liability and tax owed. Your tax liability is what the IRS keeps at the end of the year, not counting penalties and self-employment tax. If you had $5000 of withholding and get a $1000 refund, your tax liability was $4000. If you have $3000 withholding and owe $1000 more when you file, your liability is still $4000.
A non-refundable credit can reduce your liability, which will either reduce the tax you owe or increase you refund. But it can't be refunded if you have no liability. In other words, if you had $5000 withholding and your liability without the credit is $4000, a credit can reduce your liability as low as zero, potentially resulting in a refund of up to the amount of your withholding. But even if you had a $10,000 credit, your liability can't go below zero. So $4000 of the credit would be applied against the $4000 liability, you would owe no tax, and get a full refund, but the other $6000 of the credit just vanishes. (Unless it is a credit that can carry over to the next year.)
