Well. It seems yes that's the problem ..but still what you need to sort out is Why DO not having any voltage differences (data)..?
Steps you can take:
1. Make sure that the power supply to SD-card is within the voltage range.(2-3.7V)
2. Make sure that the power supply is able to provide at least 60mA to SD-card.
3. Make sure that the speed of SPI that you have configured in your program is less then 25MHz.
4. If your controller is operating at 5V, make sure you have placed level translator in between controller and sd-card.
Regards,
Salman