Why does she read my messages but never reply?

Okay so I there's this girl I dated a while back like year and half ago was when we last broke up and it wasn't a bad breakup but things happened and anyway we pretty much stopped talking she would contact me every once in a while sometimes we'd skype etc but other than that no contact we'd talk for a couple days straight then wouldn't talk for weeks month months etc anyway so we haven't talked in a good month and I text her pretty often and we both have iphones so I can see she reads my messages but she never ever replies why would she do this also she has a boyfriend so why wouldn't she just tell me to stop talking to her or leave her alone or whatever she's never said that so what is she doing any ideas or theories because I'm totally lost on what she's doing is this some kinda game any advice would be greatly appreciated

Just stop giving her unnecessary attention!

I mean not to be mean but you didn't get the hint yet she's not taking your calls, texts and messages and she has a boyfriend she's not into you anymore move on find another girl by ignoring you she's waiting for you to get the hint without her having to explain herself i don't know move on!