Jensen stereos are sold by ASA Electronics. They have a website with some help and info:
Here's the manual for operating the JWM70A stereo:
Here's a link to the product page at their website.
I'm not sure if this applies to what you are doing, but if you're using the stereo with with your phone or tablet via bluetooth you have to turn up the volume on your device then the radio volume control will more accurately respond.
Also, this stereo has it's own phone app (jControl app) so you can use your phone as a remote control.
Also, this stereo has both fixed RCA output and variable RCA output (rear) and perhaps it's been wired incorrectly and is using the fixed output. That's a long shot, but it's possible.
It has 3 speaker zones and each can have it's own volume level control too.
Hope this helps. If all else fails, call ASA Electronics tech support M-F 7am to 6pm Central time at: 877-845-8750