i'm no doc, but i do know that tolerance is a mental state. when we first start using a drug, the effects can be profound. as this profoundness wears off, they can feel less effective as the brain compensates for the altered state. the tolerance effect is most notable with mood altering substances like alcohol, zoloft, etc and narcotic-like drugs opiates, acetominophen, etc. you might recall albuterol having a "speedy" effect when you first started with it and when you do more than your usual dose that wore off after a few weeks. this is tolerance.
if you feel like it is less effective after such a long period of time, i would assume that something is different with you. like maybe there is something that is decreasing your baseline. maybe you are having more inflamation from new allergens. you said in an earlier entry that you thought a bug was trying to take hold, maybe something has.
i also just wanted to ask who put you on duoneb? i was on combivent (which appears to be the same thing) while seeing a pulmo at the county hosp, but my cf doc took me off of it immediately and put me on just albuterol. he said that atrovent (iprotropium bromide) tended to dry out secretions, thereby exacerbating the underlying problem with cf lungs. when i switched, i did notice that the albuterol didn't bronchodialate as well as the combivent, but after a few days, crap was MUCH easier to cough up, and my coughs wre significantly more productive. as a result, my baseline increased. so the alb wasn't as effective, but i was noticeably less dependent.
once again, i'm no doc, but that has been my experience. but you might consider inquiring about that, just to see what they say. i'd be curious to know what they tellyou.