Well try asking her or you can give her a few drinks get her turned on and whip it out in the heat of the moment, then say well it's not going to suck itself. This should work. I know how to treat a lady
Seriously, go into the living room during the day. Tell her -not ask - TELL her to drop to her knees and suck your cock. Tell her what a nasty slut she is while fucking her face. Paint that face. Say thanks. Zip up and go about your business. If this doesnt happen, then get a new wife.
Ladies feel free to chime in on the quality of this flinde expert marital advice.
Hmmm... Maybe plan something romantic for valentines... Get her something nice, perform some amazing oral on her first, then lay back (she should return the favor) if she doesn't, then just ask! If I had a husband/boyfriend who did this for me, he would get an AMAZING bj as thanks :-) good luck!
You've got to communicate to her how much you enjoy it and how sexy it is, and that you're not content with the current situation. Be hurt and confused by her lack of attention rather than angry and accusatory. But yeah, only communication will open the door, hopefully she walks through it.
If you're not throwing it out there regularly as something you want maybe because you don't want to rock the boat, then frankly that's part of the reason you're not getting it in the first place. We get what we settle for oftentimes. Especially in a case like this where she used to do it but doesn't anymore, it's not like you're asking her to cross personal boundaries, e.g. Anal or fetishes.
Of course this assumes she cares what you want or need in the first place. If not, well that's a different discussion.