The problem is (for me) that it's since two month I haven't received any Upvote;
conversely those who respond have 3 upvotes easily...
You've asked 4 questions in the past weeks on graphic design, only 2 of them have answers that have a score of 3. So I would not claim 'those who respond have 3 upvotes easily'. There's very little data to make that claim, and 50 percent of answers reaching that isn't exactly 'easily'. If answers reached 3 votes easily, I would've expected to see a minimum of 3 votes on all answers there.
but if you answer the question couldn't you also give me an upvote maybe?
Ideally, if you answer a question it is useful to have on the site, showing research effort and clear, and thus eligible for an upvote. Sadly, ideal circumstances rarely happen, and questions that lack either of those 3 criteria still get answered. So no, sadly someone that answers is not obliged to also upvote the question, just as they aren't obliged to actually vote down or close vote instead of answer.
So what could I do to have some Upvotes?
Write questions that show research effort, are clear and most of all: are useful. If your questions are useful, they will be found by other people struggling with the same problems, and may be upvoted by them. Patience is a virtue here.
What changes have you made to make your questions more meaningful and pleasing to the answering audience?
I've not resorted to any tricks, I just try to write questions to the best of my ability, and sometimes that's enough. At other times it isn't.
My experience on Interpersonal Skills is that you can try all you want and people will still find that 1 word that allows them to ignore your question, the limitations and the premise of it, and post an opinion on your usage of some words. If that happens, all you can do is accept you failed at writing a clear question.
On other sites, the crowds may be a bit easier or even worse. But I always try to just write a question to the best of my ability. There's only feedback on SE, and I can only get it after I've posted my question, so I just have to write and post stuff and see where it goes.