I don't use Coinbase so I'm not sure, but maybe that's issue?
Today, margin trading on Coinbase Pro provides up to 3x leverage, up to the line of credit granted to you. For example: for every $1 you put in, Coinbase gives $2 for a total of $3. If your line of credit is capped at $100, you may only borrow $100 even if 3x leverage is greater than $100.
https://help.coinbase.com/en/pro/trading-and-funding/trading-rules-and-fees/margin-trading-faq.html