Though i am not sure, but maybe their is an effect active that doesn't let you play a Spell Card.
Or have u read the requirements in the Fusion Monster card, as some cards like "Ryu Senshi" need the exact monsters (you can't use material substitute monsters for it).
To understand your problem properly, need to know what exactly you played and if possible what cards where there on the field.
I had it happen earlier. I had EHERO Heat, Lady Heat, and Neos in my hand with a Poly. I'd activated Neo Space and set a Return of Red Eyes, and suddenly for three turns straight could only set my Poly. Lost because of it. Didn't use anything on the second and third turn aside from setting a monster out of desperation. It's looking like a game bug to me most frequently found against the Vagabond.
I have one card of polymerization but i dont know how to use it. And which card to fusion
I’m trying to fusion summon Ultimate Ancient Gear Golem with Ancient Gear Golem, Ancient Gear Reactor Dragon, Ancient Gear Hydra, an Poly, but it just isn’t working. I have all three monsters on the field and Poly my hand, and it’s only giving me the option to set the card and not activate it. Even multiple turns after I set it and it still won’t let me activate it.