depends on how important the situation is. if you don't care about the person/cr, then i'd ignore it and move on.

in the instance you feel it's worth pursuing, there isn't much advice to give. be polite and open. don't act entitled, interrogative or whiny. saying something like "hey, i noticed you've been ignoring me lately. is something wrong? if there is, i'd like to be able to help." given that it's dwrp, they might not answer truthfully; if they don't then there's nothing you can do but move on and let them do what they're going to do.