My guess is that it had flowers but they may have been greenish in color and just didn't look like flowers. Are they plants that you purchased or something that grows wild?

The site linked below has info about Missouri Wildflowers and describes the leaves and flowers of four kinds of milkweed.

Asclepias syriaca has large wide leaves and pinkish-purple flowers. I've see it growing along roadsides in southwest Missouri many times. The others have smaller leaves.
I have come across milkweed in our hayfield that has insignificant greenish flowers but I'm not sure what kind it is. I think it may be something that isn't listed at this site.

Here is a link that might be useful: GrowNative.Org - Asclepias (Milkweed)

Hi,
The flowering milkweed is a swamp one for sure. It may be the one that I purchased at the Butterfly Festival, but I am not certain. I may have gotten all 3 from the same nursery even. The other 2 do not make flowers at all, but yet will have like 5 stalks of pods coming off of one main branch. They look like these 2 pics:

http://www.butterflygardeningandconservation.com/graphics/host/mw_pod2.jpg

http://www.shortcourses.com/naturelog/milkweed-swamp-02.jpg (but my leaves are more like the swamp milkweed's leaves)

Every day I clip the open seed pods off to collect the seeds for replanting, but it'd be nice if it flowered so that the b-flies and hummers would come to them.

Flowers aren't always showy but I do think you have to have flowers of some kind in order to get seeds. Perhaps it had already finished flowering when you bought it and you'll get some flowers next spring. Orange butterfly weed (a. tuberosa) flowers pretty early and DOES have showy flowers. Could that be it?

No. I look at this plant every day. It's like other milkweed flowering stems where you have one main stem and then it branches to several stems where the flowers bloom, except instead of flowers, it's seed pods - like 5 seed pods opening at different times on the one main stem. And they have Milkweed Bugs just waiting for that pod to open! The seeds are flat brown (like a flattened dried up black-eyed pea) and attached to a silky white blow-away "parachute"... to help carry them for growing. Each pod holds no less than 50 seeds. Now, the seed pod may start out looking like a teeny tiny cauliflower (if I recall), but it's nothing that produces nectar as there is nothing that comes to it - Monarchs/bees/hummers/flies flutter around my flowering milkweed all day. I will try to get a pic when I can. Monarch Watch was at our local botanical garden last weekend and I talked to a few of the workers and no one could help. Maybe I need to go back to the nursery where they were purchased! And I agree, Orange Butterfly Weed is pretty! I wish I had some!!! The leaves of this one are identical to the Swamp Milkweed (with the light pink flowers).

Hi liljaster, if these plants with the milkweed pods are Asclepias incarnata also, obviously they have already flowered and gone to seed. I can think of several reasons why they would have flowered before the other Swamp milkweed plant you have.

1) If the plants came from two different nurseries, then perhaps the plants that flowered and have gone to seed already came from a wholesaler in a more southerly location.

2) The already-flowered plants could be older than the one that is flowering now. This year my established Asclepias incarnata plants which are approx. 4-5 years old bloomed in early July. However, I have numerous 2nd year plants (started from seed in 07) that bloomed about a month later than that. The younger plants were much smaller early in the season, took longer to grow to a significant size, and bloomed about a month later.

3) If the plants are in two separate locations in a yard, it's also possible that one location is a warmer micro-climate than the other. THis can also affect bloom time. My white 'Ice Ballet' also blooms a bit later than the regular pink A. incarnata.

Can't wait to see a picture. I'm wondering if the plants bloomed early and had very tiny seed pods when you got them. Sounds like the seed pods are all maturing now. My Hello Yellow which is a variety of the tuberosa has only seed pods on it. It bloomed really early and it has taken the pods a long time to mature.

I have a similar problem. I grew a butterfly bush from seed for 3 years, it got very big. I was so excited for flowers! I looked every day with anticipation. It made seed pods but I never saw flowers! Are there any conditions which could cause this?