If a woman has left you on read, there are a number of reasons why and this depends on how long she's not replied. Either she is genuinely busy or got distracted, or it can be a bigger problem - she's not interested, you came across too needy, or she doesn’t see you as someone she wants to date romantically. If it's been over 24 hours, she most likely left you on read to avoid having an awkward “I’m not that into you” conversation.