It looks like .input-group-btn has font-size: 0; set to it and your .btn component font-sizes are set to ems. When you use ems, it’s based off it’s parents font size, which in this case is 0, so it’s not showing up.
If you change your font size to rems instead (in this case on .btn), it will show up fine. Rems are also recommended when using font sizes. Here is a good article on using ems vs rems that will do a much better job of explaining than I will: http://zellwk.com/blog/rem-vs-em/.