When we get older it's more likely friendships go through phases of lopsidedness when someone has young children, a new job, new romance or a health issue. The balance might be more one-sided while the other person is in need. And in the long run it evens out for committed friends and they are happy to take up the slack in those times.
That's all good and fine if that is happening on both ends, but what do we do with friendships we're still committed to that are not flourishing as they once did and they get way out of balance?
Unreciprocal relationships build resentment. When one person makes more effort on a continuous basis the friendship gets overly lopsided and it strains both sides. The friend who is making the greater effort starts to resent the other, the other then begins to feel put-upon; it just doesn't work. How can you maintain a friendship without feeling taken advantage of and drained by the friends who are just not that into you?
First, see if the relationship has simply run its course and it's time to let it just fade away. If that's so better to let it die its natural death. If the relationship is still worth something but you are tired of being taken for granted then it's time to backburner it. Backburning a friendship is just letting it simmer without adding anything more to it. Stop investing in the relationship or simply downgrade it. Mindy on The Mindy Project amusingly refers to friendships this way: Best friend is not a person, it's a tier. Whether with BFFs or secondary friends maybe it's time to take them down a tier or two if you find the friendship lacking.
There is no need for drama or hissy fits like in grade-school just take it down a notch and if it is meant to be rebuilt let the other person come back and make some effort. If not, then sorry to say they probably aren't really your friend anymore.
As sad as ending or backburnering friendships are there are many other people out there who would love and appreciate your friendship so now you'll have the time and energy to go find them. And don't take it personally or let it taint the fond memories you've had; sometimes we outgrow friends, gasp- they outgrow us, or we've just grown in different directions. In the end if we're meant to stay friends maybe a slow simmer on a different tier (yes, I'm mixing my metaphors) is all that is needed to eventually bring the friendship back to the top of the tiers.