Missing of image when conversion from flv to 3gp

I use in command line for ffmpeg

-i Input.flv -vcodec h263 -b 256k -r 15 -s 320x240 -acodec libopencore_amrnb \
-ab 7.4k -ar 8000 -ac 1 -f 3gp Output.3gp

The result is audio-only, without video. But when the 176x144, it works 开发者_开发技巧great.

What's wrong in using of frame size (320x240)? And what is the solution?

Are you sure there is no video in the resulting Output.3gp file? Is it possible that the end device does not support 320x240?

It would help significantly if you were to include the entire FFmpeg output in your question.