I make a H-bridge circuit on my breadboard. It will reverse polarity(i checked it with leds) but when I hook it up to my motor the motor wont run. If I use the power supply that I am using for the H-bridge to run the motor it works fine though.
Here is the schematic:
So if you know why it isn't working with the motor please tell me! 
ps: I was running the driver at 12V.
Well you diagram shows a flow of conventional current from VCC into a transistor opposing the arrow direction e.g the input and sink direction. If your transistor are in the same order, then it will not work!
Flip all the emitters and collectors and try again.
Lefty
Sorry, much approach maybe wasn't the easiest. But you I thought it would be good to help you work out the problem for yourself with a little guidance.
Hope it all goes well! 
Ben
The schematic shows the transistors in the wrong direction but in the actual build they are in the right way.
Please take a look at/on this side:
http://library.solarbotics.net/circuits/driver_4varHbridge.html
The inverter part is interesting but I still don't see why it isn't working with the motor.
What is the gain of the transistor(s) that you are using? Or what's the part number of them? It's possible that a 1k Ohm base resistor is too high.
Take out one side of the circuit and see if it works with a simple on / off. Disconnect this and then wire up the other side again just on / off but the other direction.
Note you can't drive this directly from the Arduino as you need the same voltage as the motors to drive the transistors. This applies to the ones on the top that are acting in the emitter follower mode. As Anachro says this puts reliance on the current gain of the transistor. Some large current transistors have a gain of only about 10 to 15 so 1K may be too big.
The other thing is to test it with an LED (and resistor) so you know the current is not an issue.
http://pdfdata.datasheetsite.com/web/59539/MPS-A42.pdf < that is the transistor data-sheet.
I have tested it with LED's and it worked with them.
According to the datasheet that transistor only has a maximum collector current of 500ma. That's probably not enough to power your motor. Assuming you're saturating the junction I'm surprised you haven't burnt them out.
And I don't see how you're going to make an h-bridge with 4 NPN resistors. You need 2 NPN and 2 PNP.
Yes that transistor is not man enough for the job.
You will see from the data sheet that the gain also drops off with collector current. The graph only shows the gain up to a collector current of 100mA and at top temperature this has dropped to zero.
What is probably happening is that they are conducting but not enough to supply sufficient current to turn your motor. This is confirmed by the fact that it will work if you place an LED as the load.
So you need a beefer transistor or a FET.