My return says that my credits exceed my tax payable but I am not receiving a refund for the full amount of tax paid, any ideas?

When you see this message, it means you have enough Non-Refundable Tax Credits to reduce your "tax owing" to Zero. Tax owing is what is the amount of tax you should be be paying based on your "taxable income". When this amount is reduced to Zero, any tax you have had withheld from your income (IE: Box 22 of a T4 slip) will be refunded to you.