random subtitles missing after remuxing converted PAL-to-Ntsc

[spiritgumm]

I'm converting a sample of PAL VOB to Ntsc (sample starts at the beginning of a movie). When I take the newly converted Ntsc VOB and try to add the subtitles from the PAL video (demux PAL to get two SUP files; demux Ntsc and remux with two SUP files), some subs are missing. They aren't hidden below the visible screen. The subs that are there seem synced to the audio/video. The PAL and Ntsc are the same length of time. Is there a reason why this is happening, and a solution?

[Baldrick]

have you tried with another authoring tool and see if same problem? like muxman or guifordvdauthor.

[spiritgumm]

I thought I saw a post somewhere describing missing subtitles at the start, but I couldn't find it. I was hoping it was a specific phenomena that someone might know about.
I used Muxman, which is usually fine, so I hadn't taken the next step to try another muxer yet. I did demux/remux the original PAL sample, and the subtitles were there at the start. I'm thinking something has happened during the conversion to throw off the Ntsc VOB from the original Pal subs. I'll try another muxer, and post back if it worked or not.
LATER:
I had a problem installing GUI for Dvdauthor so I wasn't able to use it as a muxer.
I used VobSubRipper to get IDX/SUB (instead of SUP) to see if Subtitle Creator would convert that SRT. I haven't successfully done the conversion, but it reported the 1st line's Index was out of bounds of the array. So I might have done something wrong with the Ripper.
I'm new to all this, so I'm just speculating: the original SUP can mux back with the original PAL video, but can't mux perfectly (at least first 2 lines missing) with the new Pal-to-Ntsc video. This seems like the issue to solve.

[u9352714]

According to this recent topic, both the timing and size of the original subtitles must be adjusted in order to be compatible with NTSC:

https://forum.videohelp.com/topic362105.html?highlight=subtitles%20pal

[spiritgumm]

The subs that are present seem to be synced correctly, which makes sense since the PAL and Ntsc videos are the same length, timewise. I thought I'd try changing the SUP framerate (from Pal to Ntsc) but the subtitle programs I used had problems running.
I didn't see anything about changing the size of the subtitles in that post...
Aside from the framerate possibly being the problem, could the Ntsc is be missing some flag or something?

[spiritgumm]

I tried changing the framerate from 25 to 23.976 (and again to 29.97), and the subtitles got unsynced and weren't all there. I used VSrip to get the SUB/IDX, then changed the framerate in VOBSub cutter, then changed back to SUP in SubToSup. Previously tried Time adjuster which reported SUP binary needed to be fixed, which it tried and then shut down. And SubMagic which just stalls.
I searched the subtitles sites to see if I could download them (possibly already in SRT format) but no luck.
When I mux with Muxman, it indicates that the subs need to be both Letterbox and Widescreen, which I checked off properly. The subtitles are all visible if I load the SUPs and the new Ntsc VOB (which has no subtitles yet) in Dvdsubedit. I imagine this problem has happened to others...