Did you by chance let them go to seed? Alliums that go to seed will use up energy stored in the bulb.

Unfortunately, a lot of alliums seem to start producing seed on the flowerhead while it's still in bloom, so to truly prevent seeding you have to sacrifice the last bit of bloom.

Don't cut off the stem (it's photosynthetic); just cut off the flower head just below the top to prevent seeding.

Aside from that, make sure they are in full sun and good soil etc... that's all stuff you probably already know.

From my understanding alliums are pretty drought-tolerant but that often means they don't do well with too much water. Was last year perhaps unusually wet? Maybe the bulbs or roots were attacked by nematodes or started to rot.

Wow, thanks SO much pizzuti. I didn't even think about how the seedheads could steal away energy. My 'Purple Sensation' always came back after leaving the flower/seed head on so it never even crossed my mind. Think it will bloom next year then? I will be sure to try cutting the head off first thing.

You are right, we did have a very wet spring this year. The alliums did send up foliage, so hopefully it didn't rot out though.

You have been a big help, thanks again pizzuti!
CMK

I also have this issue of 2 of the 3 schubertii not blooming after initially blooming the first year .. i also left the seed head on all 3 that bloomed that first year but thrn in the winter i did cut them all off but only 1 came back

Mine were the same I planted at least 20 in full sun heavy clay soil right depth came up really well first year ,well pleased but only say half dozen are flowering this year or about too ,I left the flower heads on for the winter display which keeps the interest there but wondering is that the reason they are poor this year ,any ideas or advice please