Why image background of the button is not showing? [closed]

Question

I'm writing a website and I encountered one problem. I used 'background-image' in CSS, but instead of my image I see grey square. It's Q&A section. Here is the HTML structure and CSS code (I replaced my website content with with other words, the code is the same):

.qa-background {
  width: 900px;
  background-color: white;
  border-radius: 10px;
  box-shadow: 2px 3px 5px 0 grey;
  vertical-align: middle;
  text-align: center;
  z-index: 0;
  padding: 40px 0 20px 0;
}

.question {
  padding: 0;
  background-color: white;
  border-radius: 10px;
  border: 2px solid #169149;
  display: flex;
  text-align: left;
  justify-content: space-between;
  padding: 5px 30px 5px 30px;
  margin: 0 60px 10px 60px;
  z-index: 1;
}

.question p {
  color: black;
  font-size: 20px;
}

.qa-arrow {
  all: unset;
  width: 30px;
  height: 30px;
  margin: 0;
  background-image: url('assets/img/icons/qa-arrow.png');
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  border: none;
  cursor: pointer;
  z-index: 2;
}

.qa-arrow:hover {
  filter: brightness(1.3);
}

.qa-arrow:active {
  transform: scale(0.95);
}

.answer {
  background-color: white;
  border-radius: 10px;
  border: 1px solid #bababa;
  text-align: left;
  padding: 5px 80px 5px 30px;
  display: block;
  margin: 0 60px 30px 60px;
  z-index: 1;
  animation: fadeIn 0.3s ease-in-out;
}

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

<div class="qa-background">
  <div class="question">
    <p><b>Some really nice question</b></p>
    <button class="qa-arrow"></button>
  </div>
  <div class="answer">
    <p>Answer</p>
    <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
      <li>7</li>
    </ul>
  </div>

  <div class="question">
    <p><b>Another amazing question</b></p>
    <button class="qa-arrow"></button>
  </div>
  <div class="answer">
    <p>Answer<br> some content</p>
  </div>
</div>

I have checked file path in HTML and it's correct. I have also checked linking from my HTML file to CSS file and it's also correct. When I style other elements in the document everything works perfectly fine, except this one. I have tried 'all: unset;' option, but now the button is not showing at all, only the cursor is showing. Can someone help me figure out what's wrong, please? :)