Issues attaching documents to invoices

I have 2 businesses that i use zoho for. One of my businesses i need to attach work orders to every invoice. I discovered that every time i try to attach more than one document it for some reason blocks all attachments for that invoice and all others after that. so i then have to email them separately, which is a bit annoying, but more unprofessional looking. twice i tried to handle this issue with customer service... and nothing worked until they did some sort of system update. the last time i made the mistake of trying to attach more than one document i decided to just not bother anymore with customer service. So for months i have not had the ability to attach documents. and i have tried it weekly with no luck. a few days ago i had to attach a document for my second business and it worked! when returned to invoice something for my first business it worked again as well when it had not worked just 30 minutes prior to that. so i guess if it happens again i can just fabricate an invoice for my other business. ha ha. Do we have any idea what cold be causing this issue? is anyone else having trouble attaching multiple documents?