In a recent D&D 5e Adventurers League adventure, two of our characters attempted to copy spells from each-other's spellbooks. The DM told us that each character would need to roll an Arcana check to determine success or failure in the copying of the spell.
Rules on copying spells can be found in the Player's Handbook on page 114 as well as in the most recent Adventurer's League Player's Guide (Version 6.0, effective March 24, 2017) on page 5. Here is the link.
In the AL Guide, it stipulates (emphasis mine):
Downtime: Copying Spells
If you wish to copy spells into your spellbook, you must do so using this downtime activity—even if it occurs during an adventure.
For each downtime day spent copying spells, a character may spend eight hours copying spells into their spellbook. Two or more characters participating in the same adventure together at the same table that choose to perform this downtime activity may allow one another access to their spellbook; effectively “trading” spells between everyone involved to copy as normal. Both parties to the trade must perform this downtime activity to trade spells.
As there may sometimes be a chance of failure when copying spells, you must perform this downtime activity in the presence of the table’s DM.
Here are my questions:
As the text says there "may sometimes be" a chance of failure, can the DM ultimately decide if a roll is required or not, or is there a written rule somewhere that requires it?
Are there any written rules on how this chance of failure is to be calculated?