Captain_SoulFox
I'm not sure how it works for translated novels, but all of them became locked after more or less a hundred chapters. The chapters are locked so that this site can earn from them. You must know that spirit stones is the currency here. WN gives free Spirit Stones every day but they expire after thirty days. Also, they only give a limited amount, so if you're reading many novels or chapters and most (if not all) of them are locked, you will need to buy spirit stones. Btw, bought Spirit Stones don't expire.
Btw, JacksonWalker is correct, when it comes to original novels, not all became locked, only if that novel is contracted will it become locked.