Some further info:
An MKV that did work gave this info from MediaInfo: -
General
Format : Matroska
Overall bit rate : 5 871 Kbps
Writing application : mkvmerge v2.0.0 ('After The Rain Has Fallen') built on Jan 13 2007 19:58:56
Writing library : libebml v0.7.7 + libmatroska v0.8.0
Video
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L5.1
Format settings, CABAC : Yes
Format settings, ReFrames : 8 frames
Muxing mode : Container profile=Unknown@5.1
Codec ID : V_MPEG4/ISO/AVC
Duration : 1h 46mn
Bit rate : 4 990 Kbps
Nominal bit rate : 5 234 Kbps
Width : 1 280 pixels
Height : 544 pixels
Display aspect ratio : 2.35
Frame rate : 23.976 fps
Resolution : 24 bits
Colorimetry : 4:2:0
Scan type : Progressive
Bits/(Pixel*Frame) : 0.314
Writing library : x264 core 54 svn-620M
Encoding settings : cabac=1 / ref=5 / deblock=1:-2:-1 / analyse=0x3:0x133 / me=umh / subme=6 / brdo=1 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=2 / deadzone=20,10 / chroma_qp_offset=0 / threads=3 / nr=0 / decimate=1 / mbaff=0 / bframes=3 / b_pyramid=1 / b_adapt=1 / b_bias=0 / direct=3 / wpredb=1 / bime=1 / keyint=250 / keyint_min=25 / scenecut=40(pre) / rc=2pass / bitrate=5234 / ratetol=1.0 / rceq='blurCplx^(1-qComp)' / qcomp=0.60 / qpmin=10 / qpmax=51 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / ip_ratio=1.40 / pb_ratio=1.30
Language : English
Audio
Format : AC-3
Format/Info : Audio Coding 3
Codec ID : A_AC3
Duration : 1h 46mn
Bit rate mode : Constant
Bit rate : 640 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Rear: L R, LFE
Sampling rate : 48.0 KHz
Title : Dolby Digital 5.1
Language : English
The one that didn't work had this info: -
General
Format : Matroska
Overall bit rate : 2 853 Kbps
Encoded date : UTC 2008-09-20 04:37:46
Writing application : mkvmerge v2.1.0 ('Another Place To Fall') built on Aug 19 2007 13:40:07
Writing library : libebml v0.7.7 + libmatroska v0.8.1
Video
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.1
Format settings, CABAC : Yes
Format settings, ReFrames : 3 frames
Muxing mode : Container profile=Unknown@4.1
Codec ID : V_MPEG4/ISO/AVC
Duration : 2h 5mn
Nominal bit rate : 2 586 Kbps
Width : 1 280 pixels
Height : 528 pixels
Display aspect ratio : 2.424
Frame rate : 23.976 fps
Resolution : 24 bits
Colorimetry : 4:2:0
Scan type : Progressive
Bits/(Pixel*Frame) : 0.160
Writing library : x264 core 64 r979 6d4af8d
Encoding settings : cabac=1 / ref=3 / deblock=1:0:0 / analyse=0x3:0x113 / me=umh / subme=7 / psy_rd=1.0:0.0 / brdo=1 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=2 / deadzone=21,11 / chroma_qp_offset=-2 / threads=3 / nr=0 / decimate=0 / mbaff=0 / bframes=3 / b_pyramid=1 / b_adapt=1 / b_bias=0 / direct=1 / wpredb=1 / bime=1 / keyint=250 / keyint_min=25 / scenecut=40(pre) / rc=2pass / bitrate=2586 / ratetol=1.0 / qcomp=0.60 / qpmin=10 / qpmax=51 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / ip_ratio=1.40 / pb_ratio=1.30 / aq=1:1.00
Language : English
Audio
Format : AAC
Format/Info : Advanced Audio Codec
Format version : Version 4
Format profile : LC
Format settings, SBR : No
Codec ID : A_AAC
Duration : 2h 6mn
Channel(s) : 6 channels
Channel positions : Front: L C R, Rear: L R, LFE
Sampling rate : 48.0 KHz
Resolution : 16 bits
Text #1
Format : UTF-8
Codec ID : S_TEXT/UTF8
Codec ID/Info : UTF-8 Plain Text
Text #2
Format : UTF-8
Codec ID : S_TEXT/UTF8
Codec ID/Info : UTF-8 Plain Text