To answer the original question, I haven’t noticed any particularly bad luck lately, and I’ve gone through streaks like you are now before. I don’t believe that anything is broken/bugged, but I don’t really have any convincing proof that it isn’t. Just keep trying and hope for the best, or try focusing on another character for a while if you’ve got options. Come back to this one later.
On the subject of leveling strategies - if we assume that the percentages the game gives are accurate, (there are no bugs, devs aren’t being misleading, everything is truly random) it makes no difference if you feed characters in groups of 10, one at a time, or somewhere in between. Over time it will all balance out.
If I have ten 1* feeder heroes of the same color, I can feed them all at once and get a single roll with a 20% chance of success, an 80% chance of failure, and a 0% chance of more than one success.
If I do them one at a time I get ten rolls, each with a 2% chance of success. Doing all those rolls, there’s an 81.7% chance of getting nothing. ( 0.98 ^ 10 ) Which means that you only get a skill up 18.3% of the time. But 1.5% of the time you get two skill ups. 0.08% of the time (1 in 1,250) you actually get 3 skill ups.
Over a large enough sample size, the results from both methods should be equal. As with most random events, a single person can only ever see a tiny segment of the total data set, so you’ll probably never personally see a perfect distribution. But people love to look for patterns, so we notice that when I roll for my purple heroes on Thursdays I always have better results, or that using seven heroes seems to level me up more than using eight, or any other coincidence that we notice in our tiny segment of the data. If following your superstitions makes you feel like you’ve got a better chance at leveling up, by all means go for it. It certainly can’t hurt. But it can’t help either. 