GDAX limit buy not being filled but taking $2.50 USD from my account?

I have tried 2x to buy BTC with an limit order on GDAX. 1st time price of BTC went well below my bid, but did not fill, my USD was reduced by 2.50 USD, when I cancelled the order it went back to full amount. My current BTC limit order is above BTC price and USD reduced by 2.50 USD
How do I get a limit order filled? Why is USD ballance reduced?

Any ideas or suggestions would be appreciated as I am a newbie to crypto.

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