The issue is that your figure captions are not in the main body of the document. Your example has the graphic floating and the caption is sitting in a text frame. To solve that, you need the macro to find the storyranges which might contain an entry.
Code:
Sub ReplaceCaptionStyle()
' Developed by Andrew Lockton (Guessed)
' Edited by laith93
' www.msofficeforums.com
Dim aRng As Range
' Showing all field codes (or by pressing Alt-F9):
ActiveWindow.View.ShowFieldCodes = True
For Each aRng In ActiveDocument.StoryRanges
With aRng.Find
.ClearFormatting
.Replacement.ClearFormatting
' Replace "CapTbl" with your own alternative caption style for tables
.Replacement.Style = ActiveDocument.Styles("CapTbl")
.Text = "seq table"
.Forward = True
.Wrap = wdFindContinue
.Format = True
.MatchCase = False
.MatchWholeWord = False
.MatchKashida = False
.MatchDiacritics = False
.MatchAlefHamza = False
.MatchControl = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
.Execute Replace:=wdReplaceAll
' Replace "CapFig" with your own alternative caption style for figures
.Replacement.ClearFormatting
.Replacement.Style = ActiveDocument.Styles("CapFig")
.Text = "seq figure"
.Execute Replace:=wdReplaceAll
End With
Next aRng
' Hiding all field codes (or by pressing Alt-F9):
ActiveWindow.View.ShowFieldCodes = False
End Sub