Here's another no-brainer: opening with lines that describe our protagonist as both blind and imprisoned, it's pretty obviously that Samson Agonistes is about suffering. But just in case you need convincing, the poem spends most of its time listening to various characters describing suffering: the who, what, where, when, and, oh yeah, why. Each of the various visitors Samson talks with offers a different perspective on Samson's suffering and their own. Does anyone find relief? Do they all? We'll leave that up to you.
Since Samson's problems are entirely his own fault, this story isn't about his suffering at all.
Tragedies are always about suffering. There's nothing special about suffering in Samson.