As always, check the error log. The most common problem is something like:
minivend.com 3Ex5lvta:minivend.com - [01/Sep/1997:09:08:43] simple /cgi-bin/simple > Encryption error: >
If the key you are sending to has not been signed by another (trusted) user, then you will get error returns from some flavors of PGP, which causes the problem. The fix is to route the error output to /dev/null (at least on UNIX) by changing the EncryptProgram directive in catalog.cfg:
EncryptProgram /usr/local/bin/pgp -feat sales@your.com 2>/dev/null
On Windows or other operating systems, the only fix is to sign the key or otherwise prevent the error from occurring.