Whenever I'm out on the bike on the road, I acknowledge other cyclists with a nod of the head / hand raise / or an "alright" shout.
I like to think most retro bikers would do the same. Anyway, most keen mtb'ers respond but the lycra'd up roadies rarely even pass a glance...
Why is this?
In fact, I was out on my road bike this morning, all lycra'd up, getting in an early 45mile training run for a sportive I've entered - numerous roadies passed in the opposite direction but only 2 even acknowledged me. Seriously...
Did a mountain bike and a road bike have a fairly messy divorce in the past or what??
"Roadies, what actually IS their problem?" I don't know - could it be that MTB riders have such huge chips on their shoulders that non-MTB riders just can't be arsed acknowledging their existence? Maybe, as shy sensitive types most of us are probably just scared of being laughed at for wearing Lycra by big tough gnarly guys on big tough gnarly bikes as they bounce along the tarmac.
Perhaps your local road bike riders have given up being friendly to all and sundry because no one ever waved back. Also, bear in mind that the signals can be pretty subtle if you're barrelling along the cratered roads of the UK on a pair of 23mm tyres. Sometimes it's as much as I can manage to uncurl the fingers on one hand in a cursory gesture let alone a full on wave for fear of veering into the next wheel wrecking hole.
Who knows and really, who cares? FWIW (which is next to nothing) I ride a "road bike" for commuting (and weekend rides actually) and I habitually wave to everybody on a bike - even people on BSOs who are always somewhat surprised to be acknowledged as a fellow cyclist. I find the vast majority of people respond but of those that don't most are riding MTBs. It's not an issue as far as I'm concerned and it hasn't made me think all MTB riders are ars*s or ask on forums "what their problem is".
To be balanced on this issue I have to admit that on weekend club runs I've noticed that my club mates tend to wave to riders from other clubs (unless it's the local rivals in which case its a shake of the fist!) but not so much to other cyclists. I carry on as normal and wave to everyone.
If it bothers you that much just stop waving/nodding to people on road bikes. Friendly types will wave to you anyway and you can respond if they do.