How do you know it's time to pivot?

What are some clear signs that indicate you should consider changing your strategy and pivot?

To set some context, I'm working on an product for more than a year now, mostly in spare time next to uni and some consulting work. The product is out for about six months and managed to attract 100 customers so far. Even though monthly sales have doubled in that time, I'm struggling to achieve a significant monthly revenue with it to consider promoting it to a full time business.

As a basic test, I have tried (and succeeded) to double the amount of monthly unique visitors (of the relevant audience, mostly by means of facebook ads) to my website for the past two months. However, I'm not seeing any significant change in sales due to that effort. Is that a clear sign something's wrong with my product-market fit or my marketing strategy?

Someone quoted "You have to be precise in order to gain results, nor behind neither ahead of time".

I think here is your problem, "As a basic test, I have tried (and succeeded) to double the amount of monthly unique visitors (of the relevant audience)".

Relevant :

You may be catering to those who are just fond of gaining information. They do not buy.

Plan your strategy to squeeze out the buyers from the audience. OR change your definition of yours so called relevant audience.

I think Paul Graham once said (if not, I'm sorry Paul) in order to know your product is going to kick ass, your product should make 100 people happy.

I guess it's clear that you still don't need to pivot :)