First I would double check your systems firmware. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers.

I would also replace the SATA cable as well as they do tend to fail.

Well as far as why it works with the HD and not the SSD gets into the drives SATA speed.

The original HD's Apple used in this system was SATA II (3.0 Gb/s), the SSD you put in is a SATA III (6.0 Gb/s) drive.

Think of it this way a Jeep can handle a dirt road without any problems but a Porsche would struggle over the unevenness of the dirt road.

In your case the data feeding down the cable from your SSD is at a higher rate than your cable can handle (road). So you need to replace it to smooth out the road.

So basically, the original cables specs were for SATA II not SATA III drives (or its slightly damaged). The replacement part is now spec'ed at SATA III.

Here is the IFIXIT guide to replace the hard drives SATA cable: MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement and here is the part you need: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable. Please use this newer cable as some people are still trying to sell the older version.