Error: Can't find the instance. Download a design with SRAM Object File containing t

when i perform sigtap ii ,the project don't have any problem before. but recently i add some module to it and use modelsim to simultate it ,then when i perform it again,it shows that "Error: Can't find the instance. Download a design with SRAM Object File containing this instance." what is the problem. i can sure that the .sof file is updated correctly.