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

The submarines had no enough space to take prisoners abroad. This is because it is fitted with so many machinery. Furthermore it could not carry enough sailors for crews to the captured ships. Hence it was found useless in civilian shipping. The solution to the problem was to sink the captured ship known as unrestricted submarine warfare.

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.