Check in stylesheet at around line no 1040.. do you have below line?
.qa-nav-cat-list-1 .qa-nav-cat-item{
border-bottom:1px dotted #ccc;
}
If not on line 1040 than anywhere? By defautl it should be there but if you have modified anything than add this line and it should add line.
>>>EDIT<<<
I believe you have modified something to hide question count in category list. I am not sure but that also could be the problem. In fact you can try below things and it should resolve your issue.
Try by changing above rule with this
.qa-nav-cat-list > .qa-nav-cat-item {
border-bottom: 1px dotted #CCCCCC;
}
if still don't work than use below
.qa-nav-cat-list .qa-nav-cat-item {
border-bottom: 1px dotted #CCCCCC;
}
But first one should work...