There are 2 kinds of memory; storage, and RAM (where active applications reside). Your 943 MB of storage should be enough free space, unless you have very large apps and RAM is near full. Sometimes RAM gets badly fragmented, and needs to be cleared. It's very rare, because iOS normally rearranges memory on the fly, but sometimes it can't complete the RAM "garbage collection." So you can help it along as follows:
- Press HOME once. Then double-press to bring up the Quick Launch screen
- Kill each app by flicking its screen image up. Continue until the only app remaining is the Home screen
- Press HOME
- Wait about 2 minutes for all apps to shut down
- Turn the phone off - hold the SLEEP button until "Slide to power off" appears, and do so
- Wait another minute
- Turn the phone on by holding the SLEEP button until an Apple logo appears
Try this and see if it resolves the problem. If not, you may have a problem with the app itself.