Timmyr, I have not had any problems with either yeast attenuating down. Yes, tweaking the mash schedule will have an effect on how low you can go with attenuation, but as long as you are following a normal Oktoberfest recipe, you should have no issues getting the listed attenuation.
In my process, as fermentation begins to wind down, I warm the beer up to 65F. This encourages the yeast to finish out the last little bits, and since all the replication is finished, there are very little off flavors in the beer. If you did not perform this step, then maybe the yeast does not finish out the last little bit. You may want to agitate the yeast a bit at a warmer temperature and see if you can get the last few percentage points to drop down. However, if your beer tastes OK, you may just want to let it go.
The other thing I do, with this yeast (and this is well after the beer has reached final attenuation, so this will not directly help your problem), is after the fermentation is done, I swirl the carboy gently or lift and twist the bucket a few times. Not a lot, but enough to get some of the CO2 to bubble a bit. I have found that the two yeast you have selected, provide a bit more sulfur then I like, and I have found that by having a little more CO2 off gas, this scrubs the extra sulfur out as well.
Good luck, but I am afraid it sounds like you have a stuck fermentation, but not a very serious one!