Hi FlyPhi,
for my fraternity chapter here in Germany, contact between actives and alumni is considered one of the most important features of fraternity life. That's why I understand your problem very well since my time as president. I am sure that the situation is somewhat different but maybe the following hints help a little bit.
If the alumni don't come to visit you, visit them!
Make a weekend road trip with active members and notify all the alumni of your chapter in the respective area - they will probably be happy to host you. That is much easier than finding the time to come and visit your chapter on campus, since most of them have to work and/or have family, that keeps an alumni pretty busy....
Send birthday cards at least for the birthdays ending with a 0 or a 5. See how they respond! They (the older the more) will be glad that the active chapter remembers them and might want to get in touch with the chapter again.
Invite especially successful alumni to give public talks, on campus or at your fraternity house - that will have the positive side effect to give your chapter a good reputation as being very academic....
And then, as others stated previously, try to get local alumni involved by inviting them personally with a few phone calls, make them organize a social for members only, a get-together for young and old, a family day involving the actives, whatever - it may be strange for them at first, to be called, but the more often you call the more they will think: hey, they really want me to get involved.....
Good luck, I know how hard it is,
Best, Matt
http://www.kdstv-bodensee.de