This is usually caused by marine growth in the center board trunk. Does your Sandpiper sail in salt water? If so then barnacle growth can easily jam up the works.
I'd find a long, thin, flat tool (something like a metal ruler) to use to dislodge any marine growth around the center board and trunk. Mask, snorkel or scuba equipment would also help.
Sometimes going out on a 'bouncy' day (when there are waves) can be enough to free up a snag like this. Unfortunately this is this also the kind of day when you would want your center board deployed.
"The cure for anything is salt water~ sweat, tears, or the sea." ~Isak Denesen
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