When comparing and contrasting two texts, it's important to remind yourself that there will always be a something to say. It is absolutely fine if you identify way more elements of the texts that are different than similar - the question has been asked by the exam board for a reason, and not just to trick you! They want to see you engaging with the texts in a way that shows you, as a student, are able to think about literature in a creative way, and are able to express your ideas clearly and coherently. It might help you to present your texts in a kind of ABAB structure, talking about one text first and using a connective phrase such as 'similarly in,' 'unlike,' or 'however, somewhat differently' to connect it to the next one. This will help keep your essay flowing neatly and cohesively.
When coming up with ideas, try thinking about things on a broader scale. For example, 'A Streetcar Named Desire' by Tennessee Williams and 'The Duchess of Malfi' by John Webster may at first seem like totally different plays: one's set in an Italian court in the 1600s and the other in a New Orleans apartment in the 20th century; one is a revenge tragedy and the other of the Southern Gothic genre. But really, Blanche and the Duchess function in very similar roles as female protagonists who commit acts that their society disapproves of, and as a result are severely punished. The themes of sexuality and class politics run through both, with the antagonists of Stanley and Ferdinand being defined by their oppressive sexual desires, and Antonio can be compared to Mitch and the other working characters of 'Streetcar' for being denounced for their lower class status. These are just some examples, but you can see how once you look more broadly at the two texts together, things that at first seem completely dissimilar soon appear rather not so.