In my current game as HRE (and ERE and almost all of Europe at this point), I'm trying to expand into Muslim lands, but to speed up the process I was going to try pressing claims rather than Holy Wars. This has been effective with Catholic countries, but I've run into issues with Muslim ones. Since I'm Catholic, they obviously won't accept an invitation to my court, so I have tried marrying the female claimants to my male courtiers. This has allowed me to get claimants for two of the three non-empire Muslim states remaining. However, I cannot press their claims for those states. One of them, a claimant for the Emirate of Beja, can claim individual duchies (but not the de jure ones) while the other, a claimant for the Sultanate of Mauretania, doesn't give me any CBs at all, regardless of whether or not I make them convert to Catholicism. Is this due to succession laws there not allowing female rulers? Is it because I'm Catholic? Is there anything I can do to change this, or do I have to take them down one duchy at a time?
Edit: As a response to the question above, I actually answered that question in my question, or at least as an emperor. Get to the title screen for the kingdom you want to take over, find the claimants, get one to your court, give him/her a landed title so he/she's your vassal, and then declare war on the kingdom to press their claim. As a king I tend to focus on inheriting kingdoms. You can force it through assassinations or, if you marry a claimant to a throne, you can fight a war to put your spouse on the throne of the other kingdom so your heir is also his/her heir.