There is always a possibility that dilution may occur.
There is always the possibility that your ownership percentage may be diluted as the company you invest in continues to grow and raise additional capital.
Growing startup companies often conduct multiple capital raises, and sometimes, up until a possible IPO. With each additional round of financing, the company issues additional stock to the new investors. When a startup is struggling, it may pursue the option of going bankrupt or raising more money in a "down round", which means the value of the company decreased since the last capital raise. This is an adverse outcome for both the founders and past investors as dilution occurs more dramatically in this instance. However, this result may be preferable to the startup going bankrupt. In either case, dilution is possible for any shareholder of a private company.