low voltage from USB ports

Recently I started to notice that some of my usb ports didn't respond when I tried to use them. I got a USB port voltage and amp reader to check them out and discovered that they all were putting out between 4.5 - 4.7 volts. So I was wondering if this is a power supply issue or something else.

Just as an after thought I checked the charging rate of my IPhone and it showed .7 amps charging.