How to match this format？

[formuid]

I have downloaded a HD video " B.mp4" from youtube,the information copied from KMP player is：
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp4 2
Overall bit rate : 2 504 Kbps

Video #1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3.1
Format settings, CABAC : Yes
Format settings, ReFrames : 1 frame
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 25mn 10s
Bit rate mode : Variable
Bit rate : 2 347 Kbps
Maximum bit rate : 5 410 Kbps
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16/9
Frame rate mode : Variable
Frame rate : 25.000 fps
Resolution : 24 bits
Colorimetry : 4:2:0
Scan type : Progressive
Bits/(Pixel*Frame) : 0.102
Stream size : 606.7 MiB (94%)

I have another viedo A (H.264,much shorter than B.mp4) , and I have converted A to A.mp4(720p) , and then tried to merge it with the video above by AVIdemux2.5.
But only the A ,part of the merged video ,can be played normally ,the B part of the merged is failed to display.
I guess the format of A.mp4 didn't match the B fully. I have tried several conversion softwares including MeGUI(not skilled),but all without success.
Can anybody help me?
Thanks!

[Baldrick]

Have you compared the details from the part a? And most looks similar?

You can also try join with mymp4boxgui.

[formuid]

Have you compared the details from the part a? And most looks similar?

You can also try join with mymp4boxgui.

My_MP4Box_GUI failed also.

A.mp4 converted by MeGUI is very similar to B ,one of the A's details is:

Format : MPEG-4
Format profile : JVT
Codec ID : avc1

Video #1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3.1
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Bit rate mode : Variable
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16/9
Frame rate mode : Constant
Frame rate : 25.000 fps
Resolution : 24 bits
Colorimetry : 4:2:0
Scan type : Progressive
Bits/(Pixel*Frame) : 0.008

I found that the mp4 download from youtube (not only B) can be recognized only by Avidemux,the other jioner softwares can not, this may be the crux of the problem.