Crucial own website says this memory is not compatible with your model Mac. I suppose this might be because officially your model Mac does not support 8 GB of memory.
The simplest explanation for your problem is the Mac adjusts the timing based on the the memory installed. Most likely the ADATA memory is slightly slower thus causing the Mac run the Crucial module at this reduced speed. When both Crucial modules are installed, this restriction is lifted, but this causes some other timing conflict that prevents the memory from working properly.
According the the manual for your Mac, the memory should meet the requirements given below. (The link to your manual came from this Apple website.)
- Double Data Rate Small Outline Dual Inline Memory Module (DDR3) format
- 30 mm (1.18 inches)
- 1 GB or 2 GB
- PC3-8500 DDR3 SO-DIMM 1066 MHz Type RAM
Crucial recommends DDR3 PC3-8500 memory for your Mac. I realize this module is not 4GB.
From the image you posted, I assume the ADATA memory is PC3-10600.
You bought Crucial DDR3 PC3-12800 memory. This memory does not even appear on CRUCIAL® MEMORY FOR MAC® SYSTEMS COMPATIBILITY CHART.
According to this compatibility chart, you would want the CT2K4G3S1067M pair of modules.
A good table comparing the different modules can be found here.
Note: I have aways been annoyed that the older slower memory is often more expensive that the newer faster memory.