All pilgrims asking for a Compostela must appear in person. This said, there is no time limit between when you finish your pilgrimage and when you seek the Compostela.
Volunteering at the Pilgrim Office, I have seen multiple incidents of pilgrims who had to catch a departing flight, bus, or train, return months later to claim their Compostela. The key is the properly completely credential from when you walked or biked your Camino.
The next time you are in Santiago, present yourself with the credential. Explain the circumstances of your having to leave Santiago before being able to obtain the Compostela originally.
I am certain they will issue your Compostela, back-dated to the actual date you arrived. But, do ask up front for the date adjustment. Otherwise, they will issue the Compostela as of that date.
I hope this helps.