Why were submarines considered ineffective for blockading a country and (b) what solved that problem?

In world war 1, submarines were believed to be ineffective at blockades because they were full of various things necessary for sustaining themselves so there was no room for any prisoners or similar things. They also couldn't have enough sailors to completely take away ships from others. They were also not used for civilian ships.

Germany solved it by introducing the policy of unrestricted submarine warfare. This meant that they would not take anyone prisoners and would just shoot ships on sight. They didn't think of prisoners or taking ships, just crashing them would be enough. Even those that were considered neutral or civilians would be attacked.