You can define a paragraph style with Space before specified This option is available in Indents and Spacing section in Paragraph style options. The same style should be applied to the first paragraph of every chapter. The other option is to select the text frame and set Space Before from Custom Layout section in Object Export Options dialog. The dialog can be accessed from Object->Object Export Options.
Are you exporting a book to epub via book panel or exporting a single InDesign document to EPUB with split document on the basis of styles set?
Thank you pooja2087,
I already knew those tricks. I have already applied them and... not working. Something strange is hapening...
Can you share more details on what exactly is happening and it would be helpful if you could share a test file wherein you are encountering this issue.