I really like McDonalds salads as a healthy* and reasonably priced lunch whilst I'm out but have found the same as you that the availability of them is massively variable. My variety of experiences has included:
- No-one, not even the manager-type-person in a white shirt, knowing what they were (they kept thinking I wanted a wrap);
- Staff ringing it through incorectly so I get overcharged and then have it served on a cardboard tray rather than a plastic bowl;
- Many cases of being told they simply don't have any.
On the flip side, I was amazed to find at their Greenwich branch a few years ago that they had their salads prominently advertised on the big poster-display-things above the checkouts. Only place/time I've ever seen that.
* There's been some bad press saying that their salads are actually very unhealthy. Truth is, it's entirely down to which options you choose dressings, meat content, etc.