My children seem to be overlooked for rewards all the time too.
DS2's class have multiple rewards systems. They have star of the day (every day), a class dojo system, eating on the 'top table' at lunch and getting your name in the school newsletter.
There are 30 in his class, so with the star of the day system each child should really be getting it around 6 times a year. DS2 has been star of the day once this year, and he had to share it with another boy. He hasn't won any of the weekly rewards at all this year. He's been on the 'top table' once in the 3 years he's been at the school. I think he's had his name in the school newsletter once in each of YR and Y1 and not yet in Y2. When they did star of the week in YR and Y1 he got his turn right at the end of the school year both time, and was really upset about never having been star of the week by that time.
It wouldn't be so bad, but the same names appear in the newsletter/are on top table repeatedly. And, as others have noted, these aren't the children who are well behaved and such like. DS2 finds it pretty demotivating. He is generally well behaved at school and he works really hard. He always does his homework, and any additional projects set. But he feels that the 'naughty kids' get all the rewards.
The only rewards he consistently gets are the ones where it's entirely in his power. So he's fanatical about doing his reading and getting me to write in his reading diary so he can get stamps on his cards and a certificate. In fact, he always exhausts the possible certificates by Easter (he'll have enough stamps for the top level certificate by this half term).
He can't decide on whether he gets class dojo points, or whether his name goes in the school newsletter, and he clearly feels really frustrated by it. He keeps telling me about how he does his best but just doesn't get any points for that, while others get loads of points despite breaking lots of school rules, etc.
I can totally understand the logic of having a positive behavioural rewards system and I can also understand why the children with very problematic behaviour get rewarded for behaviour other children would be told off for. But I'm not a 6 year old trying my best and wondering why I'm not getting any rewards for my efforts. I'm not sure that a system that frustrates and alienates children like my DS is actually a good one. But equally, I'm not sure how a school can 'win' in these situations.