Hey @Divyank_Mahalle,
What version of Enterprise Search are you running? Also, on the content source summary page, does it show syncs having successfully completed? Finally, anything interesting in the content source diagnostics: Troubleshooting errors during content source sync | Workplace Search Guide [7.15] | Elastic?
You mention you've tried both with and without document permissions enabled. One thing to keep in mind is that the necessary OAuth App settings in Dropbox differ in incompatible ways with relation to document permissions. See Step 7 here: Connecting Dropbox | Workplace Search Guide [7.15] | Elastic.
I hope this helps!
Ross