CAN'T RECORD software instruments with my yamaha ypt-300 in garageband

SOURCE: what cables for yamaha keyboard psr295 to connect with tascam 144

There are different options depending on what you want to achieve. If you want to send only MIDI data to your PC (for example, to run a VST instrument), then you can use a standard MIDI cable to connect the "out" on your keyboard to the "in" on your Tascam. You should also be able to use the Yamaha USB cable to connect directly to your computer, which would then bypass the Tascam (still giving you the same outcome, but without the need to buy any extra cables.
However, if you want to send the audio data (i.e. the actual sound) of the keyboard to your PC, you would simply use a standard guitar lead from the line out of the keyboard into the line in (both, if your keyboard has stereo outs) of your Tascam. This will eliminate any need for VST instruments, as you will be recording the audio signal directly.

SOURCE: Using a Tascam US122L with garageband via USB

hi, you might first want to check your performance settings by clicking the apple and clicking prefernces then setting it to high performance under the power ratings options.
also update the drivers from http://tascam.com/product/us-122l/downloads/
other than that it may be a problem with the unit. make sure if your using a laptop you are using a power supply.
also check the usb lead and compatibility of the unit.