Well in any case, and I believe I mentioned it earlier, margarine has a lot of water in it (compared to butter) and so is not as good as butter for saute, and must be adjusted for in baking when comparing to using butter or other shortening. Furthermore margarine formulation varies from brand to brand, and manufacturers may reformulate their product whenever they believe marketing conditions might improve their profit line by reformulation.
On the other hand butter is butter. Although there are brand and regional differences butter is still more like other butter than margarine is like other margarine.
I have to admit I haven't baked my favorite oatmeal chocolate chip cookie recipe for a long time. When I try it with butter, it works well, but when I try it with margarine, it just doesn't turn out the same as it used to at all.
Is margarine just not the same as it used to be?
So what I'm telling you is that I'm not surprised your recipe works better with butter, and that you shouldn't be surprised by the idea that margarine changed. Maybe you switched brands, maybe the brand you use reformulated their product.
My brand of margarine (Smart Balance) lists ingredients as "natural oil blend (soybean, palm fruit, canola and olive oil), water, ..." So you know there's more oil than water (due to the order on the ingredient list). The rest of the ingredients seem to me to be minor ingredients so I think it's safe to assume that the formulation is somewhat more than half fat (oils) and somewhat less than half water.
We can make some assumptions. Let's assume that both butter and margarine are primarily fat and water (and ignore the rest of the ingredients). My margarine has 9 g of fat in a 14 oz serving. My butter has 11 g of fat in a 14 oz serving.
The margarine is 9/14 fat = 64% fat, so it has 36% water.
The butter is 11/14 fat = 79% fat, so it has 21% water. (Probably less because I think butter has a lot more non-water composition, including milk solids).
My calculator tells me that margarine has about 70% more water than butter, maybe more. (36%/21% = 171%) That is IMO the primary reason why margarine doesn't act like butter!
I don't recall that cookie recipes generally use water. I looked up several oatmeal chocolate chip cookie recipes on the Internet and none of them called for water. If you're going to substitute margarine for butter you're adding less fat and making up for the difference by adding water. It's not surprising that doesn't work as well.