There're several ways to choose from, depending on what your app is all about. Without the finer details available, following broad methods can be explored, while if more functional details were available it would have helped to illustrate how each one of these can actually be implemented in a variety of ways across functions and features, and one can get really creative with this stuff:
- Viral marketing campaigns, which could be a long list of ways involving a number of popular social media platforms (max potential for getting extremely creative here)
- Advertising campaigns; ATL/BTL and across various channels, some at a cost, some free
- Operator tie-ups and collective promotions, yet more effective in cases where revenue-share model becomes feasible and lucrative for either party
- Cross-promotion; dig for providers that offer your target user related offerings, collaborate with them for 'I promote you, you promote me' opportunities OR draw out an imaginary construct of your target users' daily lives around places they go and things they & make your promotion's presence felt around those avenues and things effectively
This all being only about ways to promote, there could be other reasons related to your app itself (technical or ease-of-use related, and others), which could be directly/indirectly reasons why your app is not getting its due.
There could be problems unfathomed. Make sure that you are able to track those and correct, by using metrics and KPI around app functions and usage across different parts of the app, size, stability, and a whole list of things. A negative word can counter the effects of tons of your positive promotions - go even dig the internet for anything and everything on your app.