Skipped figure numbers

I am writing a book and make extensive use of the <$n:figure:name> construction to auto-number and cross-reference figures. It mostly works fine, but on occasion a figure number is skipped; e.g., it goes from Figure 6.5 to Figure 6.7. I have scrutinized the syntax and searched the relevant blocks of text for “fig” to see if there is a phantom reference in there, but no luck. Any suggestions?

I fixed at least one skipped number; I had an extra “:” in the figure name (hard to see!). I suspect that other skipped numbers are similar slight differences in the character strings.