The only time this isn't anything to worry about is in specific situations as it can depend on context.

1) The most likely - She's not interested and is being polite. If she likes you, when you invite her to meet she would make an effort. Best advice is to move on and not bother with anymore texting or messaging.

2) Far less likely - You've invited her out but she genuinely doesn't know her plans yet. She might have work or be meeting other friends and trying to see where you can fit in. The sad truth is you're a low priority especially if you've just met through something like Tinder.

This is why if I'm setting up a date I frame it as more of an invitation/order - "Let's go get cocktails on Friday". Less room for wishy washy replies then if you said "we should go for a drink sometime".

Generally, don't be oblivious and don't be hopeless. If you're messaging regularly and she genuinely seems interested and tries to set things up then the "I'll let you know" is probably genuine. If on the other hand she's replying rarely or keeps talking but every time you ask to meet finds some nonsense excuse. She's just not that into you.