New accec method

Hi,
I am trying create a new access method for the PostgreSQL.
Reading the manual, I learned that must implement 12 functions (build,
insert, gettuple, beginscan, ...).
In the begin its OK. As soon as I type the command 'CREATE INDEX' the
function 'build' is call and the index is create. When I call the command
'INSERT' all its OK too.
The problem is when I do some query. NEVER its call the 'beginscan',
'gettuple' or another function that is related with a query, that is, in the
querys my index do not being used.
What it can be causing this?

Since already I am thankful,
Thiago M. Ventura