Hi Jenny,
First off if your location is zone 3, then orientpet lilies are borderline for you. Did you have a long, ugly winter this year???
When lilies start to decline it is usually a sign that they are getting too crowded and need dividing....this can range from a few years to many years depending on how closely together you planted them,soil conditions,food, etc.
I would dig them up this fall and see what is going on....likely just need dividing for the asiatics...they are fast growers anyways.
The orientpets, I believe they suggest deep planting for them in your zone, maybe someone with more experience could chime in.
Moisture is not likely the problem and I have clay too, never checked the ph so don't know about that, but they all do fine unless they get crowded.
Good luck,
Flora

I agree with Flora. Orienpets need to be divided every 3-4 years; Asiatics every 2-3 years. I am also surpised that Orienpets grew for you in your zone. Lucky you!
Hollytone is a fertilizer for acid loving plants, not acidifier. It won't change soil's PH. Your soil is not too alkiline for Lilies I beleive. You can try to acidify it slightly with elemental Sulfur.
Acidifying Soil

Yeah I am surprised they grow for me too! In fact a few years ago (3?) I bought a package of Casa Blanca lilies from the close out bin at a big box store. Did spend much so I wasn't to worried if they didn't make it. Popped them into the ground and forgot about them. Last year they bloomed for the first time! I had to scramble through my notes to figure out what I had actually planted! Totally surprised me.

As for the orienpet lilies, when is the best time of year to divide them? They are growing, just not as nicely as they did at first.

I will have to look up when to divide the asiatic ones too, they are budding now, so I assume that later this year would be ok.

Thanks!
Jenny P

Wait till the foliage will go brown and die. For your zone early fall will probably work.