Your CSS:
[data-toggle="collapse"].collapsed .fa:before {
content: "\f067"; /*plus*/
}
requires the row to have a class called "collapsed" in order to show the plus. But you haven't given it that class:
<td data-toggle="collapse" data-target="#accordion1"
class="clickable" style="cursor:pointer">
If you use your browser's dev tools to look at the DOM, you will see that the Bootstrap library is adding that "collapsed" class whenever you click the minus to collapse (beginning with the second click).
So of course you should start the element with that class already applied:
<td data-toggle="collapse" data-target="#accordion1"
class="clickable collapsed" style="cursor:pointer">