Hi, I'd like to help you with your question.
It sounds like you have tried to stay in touch with your mother but she is too focused on her grandchildren to notice she has let her relationships with you and your siblings slip. Relationships take work and it seems your mother is not putting effort into yours.
There could be two reasons your mother has not made the effort to keep your relationship going. One, she feels more comfortable relating to the grandchildren. It is often the case that older folks find relationships with children easier and more rewarding because children make them feel younger, like when they had little children themselves. Also, relationships with children are less complicated, which may appeal to your mother if she finds her life stressful right now.
The other reason she may be neglecting your relationship is that she has a issue with you and your siblings. Was there any type of problem in your family recently? Was your relationship always good or was it poor? Was she ever abusive? All of these can affect how your mother treats you and your siblings.
What you can do is talk with her. Sit down with her without distractions, let her know how you feel. Talk with her in a gentle and non judgmental way. Use "I" statements when you talk, as in "I feel hurt when you don't call me like you used to". Let her know what you would like to have happen. For example, ask her if she would like to meet for coffee once a week. See what she says.
Another thing you could try is to join her when she spends time with the kids, if you are visiting from out of state. Visit when they are over her house and join in the fun. Take everyone for ice cream or bring a movie and dinner with you. If you both spend time with the kids, you will have something to share. It will also give you a chance to see why she might be spending so much time with the grandkids and not with her kids.
If she does not respond to your efforts, you may have little choice but to either keep trying or back off a bit. If you must do that, make sure you talk with your siblings about it, not to talk about your mother but instead to not feel alone about what is going on with your mother. You need to feel important and part of your family.
Let me know if I can help in any other way,