why don't string function work in WHERE clause?

Here's a sample:

select column1 from table1 where lower(column1) = 'some_string'

will not give me any results although column1 (defined as char[50]) has
values like
'Some_string' or 'SOME_STRING'

if for instance I say something like:

select lower(column1) from table1 where column1 = 'SOME_STRING'

then it returns 'some_string' correctly. The same thing happens with
'upper' and similar functions. Why don't they work when they are in
WHERE clause?

Dragan Matic