It sometimes helps to delete the Bluetooth pairing on the old computer first, then pair it with the new one.
You might also try pressing and holding the power button until it shuts off ... then press the button to get a flashing green light, then try to pair it.
You can also open bluetooth prefs on the old mac and delete the trackpad from know devices. My old 24 is 4' away and I needed to isolate my bluetooth components from the "other".