Why has my maple not changed colour?

I have 2 maple trees in my garden (sorry I don't know the variety, they were both here when I moved in 4 years ago). For the first 3 autumns, they both turned a brilliant red/orange in autumn just like the ones in USA. But for the last 2 years only one of them has 'turned'. The other one, which is situated only 10-12 feet away, only gets a tiny bit of brown around the leaf edges and no red/orange at all.
They both seem to be equally healthly all spring/summer.
What could cause the tree to stop changing leaf colour? :confused:

 

Maples give the autumn colours when there are warm days and cold nights, if there isn't much difference in the temperatures then they won't turn as dramatically. Perhaps your 2 trees are in a different micro climate? Maybe someone else will be able to give you a better more scientific explanation.

 

As I said, the two trees are right next to each other so climate differences don't make sense.
I have some photos, but I don't know how to display them on this site.
One difference I have noticed in the trees is that the green one has a lot of green on the bark of the trunk - could it be a sign of some disease? The leaves all look healthy though.

 

Maybe two different varieties - if you look in the sections there are detailed instructions on how to upload photos - via an image sharing site such as photobucket.

 

Without photographs it is difficult to give a positive reply
However ... Japanese maples DO require the cold weather of autumn and early winter to give of their best as regards colour change. We have been having considerable weather changes this past few years and this is affecting these trees. My own gave poor shows this past two years compared with earlier results
In GB we normally have little problem with too little water or too much sun/heat. This past two years have changed even that, with a long drought here last summer and a much hotter sun than normal
You can have a look at my own trees here
http://www.fujiyamagarden.com/MapleOne.wmv

 

great pictures whis4ey i like the video and music idea well done

 

That was absolutely wonderful - the leaf shapes, the colour, and the choise of music.

Thank you very much - I really enjoyed that - and got a few acers listed that I'd like to try

 

just looked at your pcs of maple, wonderful and the colours super.dont know how to get the music on but I am learning. have learned a lot since I joined gc. I even started my own blog.
http://philsallotment.blogspot.com/ or philsallotment(goole)

 

Actually, it`s not the cold that Maples want, it`s the warmth in Spring and Summer that they need to enable them to absorb the sugars in the soil which give them their Autumn colours. As for one turning and not the other, it could be that there are slight differences in their situations, for example, I have a client who has two Japanese Maples in the same bed, both are on dwarfing stock and they are within 6 feet of each other, one changed colour and dropped it`s leaves as it should the other stayed green for at least another month. Mother Nature knows more than we do so don`t worry too much. David.

 

Now David, THAT is a proposition I have never heard suggested before. I would be interested to hear the source of your knowledge in this direction
Vertrees in his famous book on the subject suggests
"Fall colours .. can be suppressed or enhanced somewhat by conditions.
Hardening for fall will intensify the colouration as the season advances and colder temperatures begin to occur."
He also suggests that possibly a slight reduction in moisture in late summer, or slight stress, may help to bring better autumn colour
It has long been accepted that it is the colder temperatures which help to enhance that colour. For that reason your proposition comes as a surprise
Your source will further my education on the subject, and be appreciated