First off, be careful in what you mean by "memory" here. You've got RAM that's used by running and cached but inactive apps, and you've got internal storage. I'll assume you're talking about internal storage here.
Apps don't necessarily get stored to the SD card (or "external storage" under KitKat). Some apps cannot be stored there. All apps keep a portion of themselves in internal storage. Apps also store data, including their own caches, in internal storage. Then there are the ad add-ons that may be consuming more space than in the past.
Start first by clearing app caches. Since you're running low on internal storage, I suggest doing this by hand. You may also find some apps you didn't realize you had. As you go through each one, also see if you can move any of them to the SD card. Don't go crazy over this, because some of them won't be worth the time, but it could help.
If all else fails, it might be time for a factory reset and reload of everything.