Use of subquery causes seq scan???

I need some help.  I have a query that refuses to use the provided index and
is always sequentially scanning causing me large performance headaches.  Here
is the basic situation:

Table A:
inv_num int
type        char
.
.
.
pkey (inv_num, type)
indx(inv_num)

Table B (has the same primary key)

Select *
from table a
where inv_num in (select inv_num from table b where ....)

Doing this causes sequential scans of both tables.  If I do a set
enable_seqscan to false before the query, I get an index scan of table b but
still seq scan table a.

Is there anyway to force table a to use this index (or another) and not
sequentially scan the table?

I'm running 7.3.4 on RedHat EL 2.1.

Thanks,

Chris