If you have a donor who has only given to a fund that is considered "non-tax deductible" then a contribution statement will not generate for them, through the traditional route. Statements only generate for those who have tax deductible giving.
If you want to generate a statement for those who have only paid to non-tax-deductible funds:
Go to their profile and click "giving" on the left side and then click the down arrow next to "download as excel" you will be able to produce a statement from that page. You may wish to customize the template to remove the mail-merge options for tax deductible giving as it may communicate $0.00 giving if you use your standard template.
If you want to generate a statement for those who have give to tax-deductible funds, and include their non-tax deductible:
If someone has given at least $1 to a tax-deductible fund and you would like to include their non-tax-deductible payments on a statement this can be done through customizing the statement template to include the non-tax-deductible mail merge option.