I am having scheduling conflicts with my scheduling manager. Need advice on if something is within reason or possibly not ethical?

I have been a server at the top restaurant in my city for going on 2 years now. I have been praised by management and moved up a couple of times. (Closer/excellent sections, etc. ) However, scheduling got tight so I found a second job that I made sure worked with my serving hours so I could still work what would be essentially long, hard weeks but not cost either company too much of my availability. I am taking care of myself. No spouse, no roommates, and limited assistance. So these two jobs were to help me make some savings and start investing (something I had wanted to do before but didn't have enough expendable income). However, scheduling was getting tighter and tighter. I was getting only two shifts a week. I brought it to the attention to the scheduling manager and at first she apologized and put the responsibility of "doing the best I can and trying to pick up shifts" on me. She made it seem like it wouldn't happen anymore and this was just her making errors in the scheduling for a couple weeks. So I do what is asked of me by picking up a shift that was dropped. A few hours later I get a notification from HotSchedules informing me that it has been denied because it's been given to another employee. I confront the issue again with the scheduling manager questioning why I was denied when I was told to pick up. She insists that someone beat me to that shift. The person who picked it up was sitting at about 25 hours that week while I was sitting at about 4 or 5. I question politely the fairness due to labor hours. She explains to me that this server doesn't have a second job and this is her sole income. This is where I'm questioning this sort of relevance when it comes to scheduling as it feels unfair. We are all servers and we all need money. I feel like there isn't any way this manager applies personal life agendas consistently throughout scheduling 30+ servers. So this not only felt unfair, but unethical. I feel if we were to play personal affairs we could also say that she has several roommates, has no car payment, help from her parents, etc., etc. I feel like scheduling should be based off of dependability, availability, efficiency, and lastly seniority. As of which I have never once been late, never missed a shift or have gotten my shift covered and only called out twice for legitimate sickness in my 2 years.

I apologize for all the backstory, but it seemed necessary for the short ended question. I just feel flustered and under valued. Thanks in advance if you made it through the whole thing.

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