You should use update. Actually you are creating a new tar archive each time overwriting the already existent (if) so only the last found txt will be in tar archive. So it would look like:

Code:

find ./ -type f -name "*.txt" -exec tar -uf myarchives.tar '{}' \;
tar -tvf myarchives.tar

The files are being added in the order they are found by find, not alphabetical or any other. In this case already existing files in tar will remain there even if don't exist anymore in the dir, only existent will be updated.
----------------
To include present and only present remove first the existing tar:

Code:

rm myarchives.tar
find ./ -type f -name "*.txt" -exec tar -uf myarchives.tar '{}' \;
tar -tvf myarchives.tar

-----------------
To always append and don't update existent and don't remove already non existent you can use append instead of update:

Code:

find ./ -type f -name "*.txt" -exec tar -rf myarchives.tar '{}' \;
tar -tvf myarchives.tar

----------------

You can run the exec command at the end of the find search, and redirect all found arguments at once (instead of executing an independent command for each found argument). This is much faster when there are a lot of files, but can cause problems when the list of arguments is too high:

Code:

find ./ -type f -name "*.txt" -exec tar -cvf txtarchive.tar {} + ;

--------------------
With this the maximum number of arguments is solved (thanks to jlliagre for the tip), but the tar must be deleted first (it executes the exec command more than once but much less times than the number of arguments).

Code:

rm myarchives.tar
find . -type f -name "file-*.txt" -exec tar uf myarchives.tar {} +