I just knew that if my opponent use Polymerization then I chain it with monster destruction card, he can't fusion summoning then I can barely see my opponent's hand. Is this rarely known or I'm the only one who knew it just now?
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.