It's all well and good to come up with this spiffy plot that requires the characters to delve heavily into Egyptian mysticism. They don't have any wizardly contacts, they have no access to wizardly libraries, and wouldn't have any idea where to start doing their research. There must be ways to solve the plots that the PCs can pull off. You need to keep their abilities and contacts in mind when you create the solutions to your plots. Even better, try to create plot solutions that deliberately draw on things the characters are good at, or at least delve into a little.
Players enjoy it when the spiffy little things they work into their characters get to see use.
If one character studied art history in college, then perhaps there's an NPC involved in the plot who is an art aficionado, who will be favorably disposed toward someone who can hold her own in a conversation about oil painting. There are all sorts of ways to work the party's abilities and interests into your plots, however large or small. If you develop this as a habit, players become more likely to work odd interesting abilities into their characters in later games. If those odd little abilities just languish, then you're simply encouraging them to spend more points on combat-abilities the next time around. This one is pretty simple.