As much as I enjoy Travis’ second and third albums, I never really got into their fourth, 12 Memories.
This fits my theory that some artists are just fit for one or two records but not long-term consumption.
I own dozens of records by the likes of Elvis Costello, Bob Dylan, Aimee Mann, Lucinda Williams and others. I can’t imagine any of those people releasing an album I wouldn’t want to own, even if it didn’t live up to their previous work.
But other artists lose me after an album or two even if I never stop liking what I hear. It’s as if the notion of having three, four, five, six, a dozen, records by those artists is just wrong by default.
Obviously many fans have every Travis album, and perhaps feel the need to own no more than a couple by Elvis Costello or Bob Dylan.
Perhaps it has more to do with the timing of discovery than anything else.
I had a thought all to myself
As I contemplated the moonlight
We’ve got it all, we’ve got it made
I don’t know how I got here
But I’m holding on for the crash
Pull myself out of the moon
I know I’ll never go there but
It’s shining down from up on high
We got it made, we got it made
I don’t know what we gotta make
Sooner or later we die
We’re apart, go to pieces
While I go to sleep
Look into my heart, oh baby
Don’t become a part of the past
You can be a part of the key
Don’t turn away, don’t run away
Don’t leave me hanging by a thread
Shine a little love down on me
We’re in love, make your peace
Wasting time is all you need
Walking down this hill tonight
I had a thought it was my own
Moon swept by the clouds and save the starlight
Or shall I be or shall I not be
I don’t know I just don’t
Feel like it tonight
Meet Me In Montauk 
Well, I think that one factor as to whether one wants to own everything of an artist’s discography has to do with simply whether that artist enjoys “favorite artist” status in your mind. So, for you, that would explain your need to own every Dylan, Costello, Williams, etc record.
I think another factor though, at least for me, is the diversity of sound from the artist. So, for example, Travis, may not hold as much interest ultimately because of a sameness in sound/style from album to album. This certainly explains the lack of desire to continue to own new indigo Girls, Tracy Chapman, etc albums. When you are in the mood to hear their sound, you go for the classics and feel less need to hear more of the same.