Unfortunately not all apps can be transfered to the SD card. That has nothing to do with how full or empty your SD card. It depends on who created the app. Some might allow, some might not. Plus, if you have less than 20mb free of internal memory, you will start having the problem with not being able to open apps, gallery, listen to music (I know it sucks)
Now I'm not sure I understand clearly that the problem is. I understand that your internal memory is full (Which unfortuantely is a huge problem in WFS devices). But was your SD card also full?
Any pictures, videos, musics that you install usually automatically install on the SD card. So I'm surprised it told you it was located in the internal memory. Any apps and the related data involved will usually get installed in the internal memory. Even if one app can be transferred to the SD card, only a portion of the app will be moved, or the app will not be able to function. So I think the problem you are facing is more of an internal memory problem than an SD card problem.
There are a few ways around the internal memory problem. The best one in my opinion, is to root the phone and install Link2SD (I did that and I have like 20-30 apps and still have about 40-50mb free)
Here's a guide on how to do it:
Or you can force apps to be installed on the external memory (Never tried it though so can't really help you there)
Forcing Apps to install to the SD Card without root