if you change the metrics you have to change it on all routers. if the K values don't match you won't get neighbor relationships in EIGRP.

the 2900 is L2, so it's probably a L3 problem on the 6k. Is PIM enabled?

right! foolish me, I stand corrected.

what kind of sup (base, 3b, 3bxl, 3cxl) and how many entries are in your FIB and TCAM?

It should be noticed that link cost matters. Most protocols won't balance over unequal links.