There are several possibilities here, the first of which is that the metadata for the Btrieve Data files which is contained in the DDF (Data Dictionary Files) is not very accurate. Most applications which use Btrieve and Pervasive.SQL 7 or 2000 do not need DDFs becuase they use the Btrieve or transactional interface(s) which are very fast and powerful, but do not allow 3rd party programs to fully access the data without DDFs. If the DDFs are old they were probably created with tools like Xtrieve, DDF Builder, or DDF Sniffer and may have problems. It is usually easiest for the original creator of the application to update these files but that is not always possible. There are tools in Pervasive.SQL 2000i (the latest version of Btrieve - 7.9) for checking the DDFs and quite a bit of information on the Pervasive Software (formerly Btrieve Technologies, Novell, and SoftCraft) Web/FTP site http://www.pervasive.com that can help you update the DDFs.
The second possibility is that the drivers don't match the engine. For instance ODBC 2.04 did come with Btrieve 6.15.451 and both were the last releases of Btrieve 6.15, but if the Btrieve client interface DLLs (wbtrv32.dll & w32bticm.dll) that the ODBC 2.04 is loading are earlier versions then strange problems can result. Also since Btrieve 6.15.451 was released just before Windows 98 in April of 1998 it wasn't fully tested or supported on Windows 98 or any later OS versions (meaning there were no code fixes for any issues on 98 or later) though I know of no 'show stopper' problems. More likely, though not in this case, the ODBC 2.04 drivers might be connecting to an earlier patch version of Btrieve like 6.15.430 and even those components may have mismatches or even some old 6.10 Btrieve components from Novell in the mix to complicate things.
Lastly, though I can get almost any Btrieve 6.15 installation working properly, it's often easier and certainly more supportable to just load the latest version, Perasive.SQL 2000i, from Pervasive which contains a complete Btrieve 7.9 as well as a full native ODBC relational engine. The nice thing is that this version fully supports all Btrieve applications and data natively and except for a few cases where there are often application bugs, you can simply plug Pervasive.SQl 2000i in where Btrieve was, rebuild the files to at least 6.x file format (not necessary but highly recommended) or later, if necessary, and keep running on a fully supported database engine that is tested on all the lastest hardware and OS vesions so that if there are issues you can get help from folks who know the products and environments.