Without knowing the model of your filter, and seeing the schematics, I can't do much more than guess.
But while broken laterals will cause the problems you describe, it's also common to see problems resulting from an overlooked or un-repaired air relief arrangement.
Most sand filters have some sort of arrangement -- usually involving small low-grade tubing which can get brittle -- that allows air caught below the sand to the top of the filter, without bubbling through the sand. Often, when sand is replaced, these tubes or gizmos will be broken or removed without even being noticed. When this happens an opening is created, somewhere below the sand but upstream of the lateral hub. You might want to consider if this might be your problem. If it is, air bypass is usually not essential, so you may be able to 'fix' things by plugging the hole, even if you can't replace the bypass with new bits.
Otherwise, a cracked hub or lateral is a good place to look. The ABS plastic used is easier to mold (I assume) than PVC, but deteriorates over time and can get so brittle it will crack if you just look at it wrong. For this reason, some pros will only 'repair' an old sand filter by gutting it, and installing all new internals.
Good luck.
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