There are two types of investors that participate in a fund: retail and institutional investors. A retail investor is an individual. Investing in a fund is usually a small part of their activities. An institutional investor generally has a dedicated team that looks at investments. Some people call them sophisticated investors.
Retail investors usually invest small amounts (in the order of $10K-$100K USD). Their investment process and the information they ask for varies wildly. Some will invest purely off an email or a recommendation from a friend. Some send pages of questions and ask for multiple meetings. These investors can be very difficult to manage, and take up a lot of time, relative to what can be small investment sizes.
These difficulties mean that many funds prefer to focus on institutional investors. This is especially the case for large fund sizes, where it is not feasible to deal with the hundreds or thousands of investors that would be required to raise a multi-hundred million dollar fund. Institutional investors usually have a prescribed due diligence process. Some do this in-house, and some use external advisors. They will ask detailed questions, check documents carefully, and take their time before deciding to invest.
The main types of institutional investors that will invest in other funds are pension funds, sovereign wealth funds, asset managers, family offices, and fund of funds (a fund set up to invest in other funds).
The fundraising process is painful and can take months or even years. It is difficult to locate suitable investors and difficult to progress them to an investment decision. This is especially true for a first-time fund without a track record.
Appointing a fundraising advisor can be helpful, especially if you do not have many contacts of your own. It is hard to contact many large institutional investors, and advisors can help with this process, as well as navigating the various investment processes of each fund.