Everyone has a history of friendships, including ones that have lasted a lifetime and ones that have failed or just faded out over time. Friendships grow and change and that can result in changes in the structure and dynamic. There's no science per se to how friendships work and are successful, some just fail, while others last and continue to be there throughout your life.
Many think that a successful friendship is one that includes individuals being connected all the time and always being involved in each other's everyday lives. While that is the case for some friendships, that's not necessarily how they all have to work to stay relevant and lasting. Like romantic relationships, everyone is different and has different needs that friends see and therefore can understand and work with. Not everyone needs a particular friend there all the time, but that doesn't make them any less of a close and genuine friend.
Some friendships are like a wave of sorts. They can have a period of time where each is very involved in the other's life and communication is constant. Then there may be a gap in that consistency where communication is more about non-every day things, such as important events or big announcements. Eventually, once that gap comes to an end, the friendship goes on as though they have been in contact every single day. They are able to pick up right where they left off, almost as though there was no time apart.
Each individual person is different, so why shouldn't friendships be? Just because it's not the traditional kind of friendship that everyone expects and that we see portrayed in movies and TV doesn't make it any less of a friendship. Each is different and some may not be as consistent, but that's okay.