You cannot change your parents. Accept that this is the way they are. You can change your reaction to what they are doing. Work on not taking it personally. Once you're able to talk with your mother without feeling hurt or the need to express your hurt, try talking with her again and ask her if there is a way that you could arrange time for your boys to visit with them that would be more comfortable or easier for them. Tell her that you'd really like for this to happen but it's OK if it doesn't.
Tell your boys that this is just the way Grandma/pa are; that you'd like to seem them more but it just isn't working out that way right now. Remind yourself and them that their grandparents love them even tho they don't see them very often.
There are many reasons, other than not wanting to see you or their grandchildren, for them to act this way. Have you tried asking your mother why this is happening? You'll need to ask in a non-judgmental, calm, non-accusatory, manner to get an honest answer. If your mother is usually a closed person, not sharing her feelings with you, she may not be able to tell you why. And if you've expressed your hurt in an angry way she may be feeling defensive and not wanting to talk about it. If you've been angry with her she may have decided to be uninvolved. Your anger is understandable but many people don't know how to handle anger or dissension.
You didn't tell us their ages, the status of their health, whether or not they're employed, how active their social lives are or how far away they live from you. All of those things can influence how much time they spend with you. It's also important to take into consideration how close you were before the boys came along, whether or not they enjoy children, especially children the ages of your boys.
Yes, it would be great if your parent were more involved with your boys. However, you cannot make that happen by insisting that they get involved. I'd try stepping back and not even ask to go over or invite them to your house. I would continue to make friendly phone calls and send e-mails if they write e-mails. You could send pictures. But just stop trying to make them change. You might be surprised that without you insisting and with you showing your love and acceptance of them as they are that they may decide on their own to get more involved.
Do you visit with your parents without your children being involved. I discovered that my mother was jealous of my daughter and then my granddaughter and the amount of attention I paid them which required that I pay less attention to her. She died a few years back and every once in awhile I wish I'd found a way to spend more one on one time with her. We were close before I got my daughter. Not so close after that. Not only because of jealousy but also because of failing health.