are you writing it correct?
@import url("/_layouts/customcssfolder/mycss.css");
@import url("/_layouts/customcssfolder/myhome.css");
@import url("/_layouts/customcssfolder/styles.css");
EIDT
how may imports are you using in a single style sheet? It does work with sharepoint as iv done it plenty of times even with sharepoint 2007!
the issue is to make sure its written properly! and that it has no more than if i remember correct 25 @imports per page, by this i mean:
you have your mast style sheet, it contain 50 @imports, anything above the 25 mark doesnt load so you would split it into 1 master 2 sub master styles and thoes two hold 25 each, you than have room to add another 23 sub master style sheets :)
I came accross this issue at work and this was my solution to the problem that is still used today as the company has over 500 clients and each client has its own branding:
so it would be:
current setup:
1 (master style) x 25 (@imports)= 25 max @imports
my change gives you:
1 (master style) x (25 (sub master style) x 25 (@imports)) = 625 max @imports
why would you do the above method? if your a company that offers a service... now you offer packages to resellers, each reseller has 100's clients and each client has their own branding! so the master would be the company, sub master are clients and resellers, and the last part are only clients... you could keep on building the tree and get more than 625 but the point is @import leaves cleaner css branding on a massive scale due to resellers and clients... each sub master would have many clients!
but if you only have a few than im sure its todo with how you have written it! can you show an example? how are you writing the url if its in the style library, you need to have the correct url to reference to that area regardless of where the mast css file resides!