Perhaps an obvious one, but I attempted to do a cvs up on a directory, and nothing happened, even though there had been changes in the repository.  After being confused for a while, I noticed that there was a sticky tag:

Working revision:    1.66
   Repository revision: 1.66    /.../.../...
   Expansion option:    o
   Commit Identifier:   2104f3e32632ef7
   Sticky Tag:          1.66
   Sticky Date:         (none)
   Sticky Options:      o
   Merge From:          (none)

cvs up -A sorts this out:

cvs up -H
up: invalid option -- H
Usage: cvs update [-APCdflRp] [-k kopt] [-r rev] [-D date] [-j rev]
    [-I ign] [-W spec] [files...]
        -A Reset any sticky tags/date/kopts.
        -P      Prune empty directories.
        -C      Overwrite locally modified files with clean repository copies.
        -d      Build directories, like checkout does.
        -f      Force a head revision match if tag/date not found.
        -l      Local directory only, no recursion.
        -R      Process directories recursively.
        -p      Send updates to standard output (avoids stickiness).
        -k kopt Use RCS kopt -k option on checkout. (is sticky)
        -r rev  Update using specified revision/tag (is sticky).
        -D date Set date to update from (is sticky).
        -j rev  Merge in changes made between current revision and rev.
        -I ign  More files to ignore (! to reset).
        -W spec Wrappers specification line.

The version in the repository was 1.68, but the file was stuck at 1.66 for some reason.