Hello!
If a player hasn't accepted your request within a few days, it's probably because they're inactive. Withdraw your request and send out a new batch. Remember that your total number of friends, pending, and sent requests can't exceed 50 (iirc).
Like Sakuragawa, if you share your ID on the forum, I can add you. I still log in every day to collect the bonuses, greet my friends, and spin the gacha.
Messaging in Cybird apps is more like a greeting log. It lists players who have greeted you and also their messages if they sent any. That definitely helps to get you noticed.