I have the tissue-specific RNA-seq data and its corresponding ATAC-seq data. I first identify all the differentially expressed genes. Then using ATAC-seq data to check whether the promoter regions of those genes are differentially accessible. It turns out that only a small portion of differentially expressed genes' promoter regions are differentially accessible. I try to find explanations. One reason I can think about is that the data is not perfect so missed some signal. What could be other reasons in the biology perspective?