You probably have one of few things going on:
1) it is on another invoice and thus cannot be added to a second invoice.
2) you deleted an invoice and the Status was Billed or Paid. You will need to open the Expense and scroll down to the bottom and make sure that in the Invoicing section that the Invoice # does not say "deleted". If it does, you will have to tap on the "Unlock Expense" button. If there is an invoice number in there, then that expense is already on another invoice and you should *not* unlock it from that other invoice.
3) you have removed the checkmark from the Reimbursable field on the expense. Tap on the Expenses tab and then open the expense. If you scroll down to the very bottom, there is a section that says "Invoicing". If there is an invoice # XXX filled in, then it is on another invoice. If that section is empty, then you probably have turned off the attribute. Scroll to the Attributes block and look at the Reimbursable line. If the line is *not* checked, then it cannot be added to a invoice (because it is non-reimbursable). So tap on that line to add the Checkmark and then Save. If neither of those are the case, check to make sure the that Client on the expense is the same as the Client on the invoice.
