The only restriction that I'm aware of is you can have up to 500 total contacts so unless you've exceeded this amount overall this would prevent adding any more.
If not please provide the mail account in question and I'll investigate further.
Hi Sydney, I can see there are 498 contacts in total so may be this is the cause. Is there a way to extend contacts limit?
Unfortunately not at this stage as the limit cannot be adjusted.
I'll pass this on as a potential enhancement request.