Why do columns in Bootstrap rebuild unevenly?

Question

css

After adding new .col-sm-3 columns to the site, they began to rearrange unevenly when narrowing the screen when there should be three columns in a row. Screenshot issue:

Answer 1

Three columns in a row is col-sm-4 and col-sm-3 is four columns in a row.

Answer 2

Do it like this.
.block2 .col-sm-3 {
min-height: 400px;
}
Or wrap each line in .row.
And when you make up, I advise you to set the background-color of the blocks, you can immediately see what, where and why it gets in.

Answer 3

.block2 .col-sm-3 {
text-align: center;
min-width: 280px
min-height: 409px
margin-bottom: 80px;
}
make min-height bigger. In general, .block2 .col-sm-3 is not very correct. It's better to add another class to .col-sm-3 so as not to touch the base classes of the bootstrap