Due to low inflation, over the past two years increases in Social Security have been small or non-existent. However, this does not explain a sizable reduction in payments.
There are a couple possible causes I can think of.
First, many states supplement the federal SSI payments. It's possible your state reduced its supplement.
Second, if your father had income or "resources" (savings, vehicles etc.) over certain limits, his payment may have been reduced.
Contact the SSA directly if you believe the reduction is incorrect.
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