a hint:
I'm also often very angry if i can't find the the following parts of a video in the "related column".
That's Youtube's weak point.
But I found out how I can solve this problem:
If you have for example video: PC - John Lear tells all Part 3, but in the column there is only part 5 ,7 , 8, 9...
but not the damn 4th part.
Then simply copy "PC - John Lear tells all Part", paste it into the "search" field and then you will find all parts.
When i want to find all parts of a certain video at the outset, then I sort the searching results not by relevance but by date, after the searching process.
And then you have all in a fine order:
PC - John Lear tells all Part1
PC - John Lear tells all Part2
PC - John Lear tells all Part3
PC - John Lear tells all Part4
PC - John Lear tells all Part5
....
and not 3, 4, 1 , 9....