Yeah, elementals are random, but you can increase the opportunities of receiving an elemental by skipping chests. I track all of my unfarmable mats, and all of my elementals. I thought I wasn’t receiving very many until I started keeping track and skipping wait times. Now, I get an elemental as early as every week, to as late as every third week. I skip wait times and fill two to five chests a day. I would suggest speeding up wait times if you’re looking for more elementals or mats, since it’s fairly cheap, and chances are you could fill three monsters and two raid chests a day if timed correctly.
Fill your monster chest in 1-8-7 until it’s full. Then, wait for your flags to hit max, skip the chest wait time, and do it again. (13-6 also works if you’re looking for 3* mats over recruits).
With raids, I’ll only skip that chest if I fill my first one fast enough, so that usually means only losing one raid per chest (at least for me). If you have to, you could start on a second raid chest. Then, finish it the next day, allowing you to complete another one later that day when your flags are full again.
However, if you do not want to skip your chests, then there’s nothing more you can do other than fill your chests as soon as they’re available, and continuing this process whenever they reset automatically (12 hours after opening your previous).
Back when I didn’t skip, I got an elemental every two to four weeks. Up to you how you want to proceed, but I’m a big proponent for skipping chest wait times. Especially when you have the flags to burn, you might as well get credit for it