So this is one of the unfortunate things about resin. Sometimes when it gets heated in the sun, it will soften. If the resin has cured hard again, I would take a piece and test either alcohol or acetone on an inconspicuous spot. If that doesnt work, you might try soaking the piece with some water and detergent.
Im also going to reach out to tech support of some of the companies we work with and see what their suggestions may be.