which version of SQL Server? (2000 or 2005)
what the index plan says?
what type of comparison do you do?
what the index tunning wizard says against your query?

comparing date/time is slower then comparing integer.

In addition to the previous post, my guess is you use the columns inside an
expression, so they are not Searchable ARGuments (SARGs) anymore. Try to
rewrite the queries to have the indexed datetime columns without an
expression in the Where clause. For example:

create table a
(a datetime)
create index aa on a(a)
insert into a values ('2006-01-16')
insert into a values ('2006-01-17')
go

select a from a
where datediff(dd,a,getdate()) < 2  -- this query should do an index scan

select a from a
where dateadd(dd,-1,convert(char(10),getdate(),112)) = a  -- this query
should do an index seek

However, take care you get correct results - don't forget that you always
have time part in the datetime data. You should check if the Between
operator would be useful for you.