Android ffmpeg - 3gp to mp4(or avi) videos - not playing -

i doing android application used merge 2 videos , after merge completed play result video. using ffmpeg.

when record 2 mpeg or mp4 videos, result merged video playing fine. record videos 3gp , convert mp4(or avi) , merged it. result video not playing.

can me may problem?

i using following code:

string args[] = {"ffmpeg","-i",""+filename[i]+"",                             "-qscale","1",                             ""+environment.getexternalstoragedirectory().getpath()+"/movies/i"+(i + 1)+".mp4(or avi)"}; 

to merge videos:

string args[] = {"ffmpeg","-i","concat:"+concatpath.tostring()+"",                               "-vcodec", "copy" ,                               "-acodec" ,"copy",                               "-vbsf", "aac_adtstoasc",                               ""+environment.getexternalstoragedirectory().getpath()+"/movies/3.mp4"};