I typically notice just poor brake performance on unkempt rear brakes, but you could have a leak or a lot of air. Check the bleeder nut on the caliper for good sealing and being tightened all the way (but don't crank it, its a gentle tight).
After that, consider your actual caliper health, it could be improperly seated, allowing a sort of "springing" due to misfitted parts.
So brake calipers by design are a 'floating' system, they can move, they have to move in certain directions during use and wear to stay a good tight fit against the rotors. If you go wiggle your actual caliper, it will move a bit. The caliper itself has 2 smooth metal pins that the caliper slides upon, and me being lazy in the brake job didn't take them out, just threw in new pads. Bad choice. Turns out the sliding motion of the caliper along those pins was not allowed at all, because the pins had dry grime totally seizing them in position. Completely solved this problem by removing the pins (with quite some force) fully cleaning them with rubbing alcohol, and applying fresh brake grease. Made my rear brake go from just about zero effectiveness to 100% instantly.
Id check your fluid levels as mentioned before, and check your lines. then lastly, Id check your actual brake pads, rotors, and the caliper and its pistons. Definitely worth the full tear down when I did my brake job, and I couldn't be happier with my new pads, fresh fluid, and freshly greased parts. If I was you, I would tear the whole system down and clean it up and inspect it if you're up to the task.
Hope that helps a bit.
Absolutely critical when doing any work on the brakes.
Some more info from Ninja250.org - https://faq.ninja250.org/wiki/How_do..._brake_pads%3F