One thing that can bite you if you're not careful is if the ruleset of your service level rule or your escalation activity is different than the ruleset of the work class. The system is only going to make sure it uses an access group containing the ruleset of the work class, so if the ruleset of service level rule and the escalation activity is not included in that access group's rule-applications, the system won't be able to find your service rule or escalation activity.
The simplest arrangement is to have your work class ruleset be the same one as the one you store your service level rule and escalation activity in. I've had situations like this where my solution was to make a subclass as a new work class so I could put that subclass in the same ruleset that I already had stored my service level rule and escalation activity in. Actually, in my case, the system wasn't even able to find my flow rule. That is subject to the same quidelines mentioned above. /Eric