I can't get combos to work, and I'm not sure why. I tried to put Shield Wall and Stoneskin together. I went to a combo master, put them together, and it shows up in my combo menu. I can equip it, but I can't cast it. It just stays grayed out. I don't know what I'm doing wrong.

Combos take a much longer time to regenerate, especially the higher the levels of the spells are and the more spells are combined. Also they dont benefit from different bonuses that decrease spell regeneration times. Maybe the combo just takes a really huge amount of time to regenerate. Hover over the combo spell and you will see how long it will really take.

So the solution would be to wait until the spell is regenerated, drinking yellow potions which immediately refresh combos or not using them at all.

If you are interested in how the spell regeneration times work, have a look at this topic: http://darkmatters.org/forums/index.php?/topic/3802-ca-and-spell-regeneration-formulae-base-value/

I think it is worth noting that when you first create a combo you have to wait for the entire cooldown to come up. It doesn't come ready to go.

As an example of the difference in regen times ThomasD mentioned, take the combo I just created: on my character Blazing Disc 6 (Daemon spell) has a 5.5 second regen time. You would think a combo that casts this spell 4 times would then have a regen time of 22 seconds, perhaps a bit longer. It actually takes 219.4 seconds! For this reason I never really found a use for combos in Sacred, Sacred 2 handles them a lot better imo.