Charitable organizations are having trouble holding onto their donors, according to the Fundraising Effectiveness Project.

The donor retention rate, the percentage of donors who give in succeeding years to the same organization, has hovered in the mid-40th percentile for the past decade, underscoring just how difficult it is for nonprofits to keep donations flowing from their supporters, the organization reported in a study released Monday.

Overall giving to charities increased 2 percent from 2016 to 2017, but the retention rate was 45.5 percent, according to the Fundraising Effectiveness Project, which was created by the Association of Fundraising Professionals and the Urban Institute. The report is based on the Growth in Giving database, which tracks 125 million donation transactions.

“While the overall growth in giving of 2 percent is positive, the millions of donors who do not repeat their giving is very concerning,” said Erik Daubert, chair of the project. “The fact that nonprofit organizations are losing 54.5 percent of their donors from one year to another is not a sustainable strategy.”

The report showed the average gift amount rose by 1 percent to $1,037 in 2017, but the average gift to small nonprofits, those with $100,000 in contribution income, declined 8.2 percent.

“What we’re seeing in general is textbook fundraising—larger organizations faring better than smaller ones,” said Steve Birnbaum, vice president of SofTrek Corporation, a contributing data partner to the project. “There are always exceptions, but larger charities, with more available resources to direct towards fundraising, will typically do better than small ones.”