AddNew button icon is not showing on screen when using FontAwesome icons

I am using fontAwesome icons for edit,save,delete,cancel .But only the addNew icon i.e(plus symbol) is not showing on screencouldn't show icon for AddNew button.If I use the plus icon inplace of save/delete/edit it is showing up but the problem is with addNew .Can someone please give me a solution to fix this?
I used the following settings
this.settings = {
add: {
confirmCreate: true,
addButtonContent: '',
createButtonContent: '',
cancelButtonContent: ''
},
delete: {
confirmDelete: true,
deleteButtonContent: ''
},
edit: {
confirmSave: true,
editButtonContent: '',
saveButtonContent: '',
cancelButtonContent: ''
}
}