Copied about 1000 songs onto a SD , about 10% in folders the rest individual files. Only those in folders and about the last 20 individual tracks play. I've noticed that all the ones that don't play have '.mp3' (dotmp3) after the track title
Sounds like they may be misnamed. That would cause the device to not read them correctly and possibly not play them.
Your best bet is to put them on a computer and check for the true file type. They may not actually be MP3 files (which should work on the phone). They could just have been named that. Not a shock. Happens a lot.
You can get yourself a program that can convert them. Your best bet would be to convert to another type (since they are already named with that extension) and then convert the new ones back to the other file type. So say convert the files to say a WAV file type (warning they are big) and then convert the WAV files back to MP3.
Me, I like the program Format Factory, but there are lots of them.
But before doing that I would test them on the computer to see if they work. There could be problems with the files other than being misnamed.
Back home and decided to remove all the music from the SD card and copy and paste it again from my laptop. My laptop confirmed they are all mp3 files. A few things have happened. 1. Some 20% of the files that seemed to have initially transferred onto the SD card ok later vanished completely - a few folders and all single tracks that are alphabetically after Satisfaction 2. More tracks actually play on my phone than before but probably half don't. 3. Of the tracks that don't play on my phone, I can play them directly from my laptop music ok but they won't play when I try and play the same files on my laptop directly from my phone SD card......