During combat, I could not use the bonus spell action of a level 4 cleric / life in some circumstances, without any obvious reason.
The 3 spells (Healing word, Spiritual weapon, Shield of faith) were greyed-out, but no reason explained.
I then switched from a one-hand ranged weapon to a one-hand melee + shield, and the reason became "need a somatic component" so that was normal. In the next turn, I switched back to the 1-hand ranged weapon and then, finally, the spells were available.
=> Found the explanation:
Bonus Actions
Various Class Features, Spells, and other Abilities let you take an additional action on Your Turn called a Bonus Action. The Cunning Action feature, for example, allows a rogue to take a Bonus Action. You can take a Bonus Action only when a Special ability, spell, or other feature of the game states that you can do something as a Bonus Action. You otherwise don’t have a Bonus Action to take.
You can take only one Bonus Action on Your Turn, so you must choose which Bonus Action to use when you have more than one available.
You choose when to take a Bonus Action during Your Turn, unless the bonus action’s timing is specified, and anything that deprives you of your ability to take Actions also prevents you from taking a Bonus Action.
But also:
Casting Time
Most Spells require a single action to cast, but some Spells require a Bonus Action, a Reaction, or much more time to cast.
Bonus Action
A spell cast with a Bonus Action is especially swift. You must use a Bonus Action on Your Turn to cast the spell, provided that you haven’t already taken a Bonus Action this turn. You can’t cast another spell during the same turn, except for a cantrip with a Casting Time of 1 action.
What had you done with your actual action?
If you cast a spell as an action and it was not a cantrip you do not have the ability to also cast that spell as a bonus action. The only reason you can do that is to allow casters to still do a mundane attack while you're casting a bonus action spell.
If the spell requires a material component and you don't have the option for those equipped you cannot cast the spell.
If the spell requires a somatic component and both of your hands are full you cannot cast the spell (I don't think they have included a warcaster feat, so the only way around that is with a battle cleric). Casting works fine with a 2 handed weapon like a greatsword or a bow.
What had you done with your actual action?
If you cast a spell as an action and it was not a cantrip you do not have the ability to also cast that spell as a bonus action. The only reason you can do that is to allow casters to still do a mundane attack while you're casting a bonus action spell.
If the spell requires a material component and you don't have the option for those equipped you cannot cast the spell.
If the spell requires a somatic component and both of your hands are full you cannot cast the spell (I don't think they have included a warcaster feat, so the only way around that is with a battle cleric). Casting works fine with a 2 handed weapon like a greatsword or a bow.
Yes, that's what I wrote in my post update the other day, just above...