I now suspect that after being wounded on the first day, and having his flagship heavily attacked, that Robert Blake was reluctant to press the fight to the Dutch. At the end of the first day, after having gone on the attack and having been repulsed, the Dutch went into defensive mode.
The Dutch formed a crescent around their convoy, and facing the English. As it was, English frigates picked off merchant ships. On the last day, two warships were also taken. The Dutch had been reduced to about 30 warships that still had ammunition. If the English had pressed an attack, they could have destroyed the remnants of the main Dutch fleet, and taken the convoy. Instead, at nightfall, the English let the Dutch escape. There was a threat of a storm and they were near the French coast. The English pilots were reluctant to enter the shallows, while the Dutch, with their shallow draft ships used the shallows to escape.