It would appear that it simply didn't occur to them to try a seventh symbol. It wasn't on the 6 symbol cartouche and while the dialling program system appears to be working after a sixth symbol, nothing then happens.
Jackson wasn't brought in specifically to find the seventh symbol, he was brought in to decipher the capstone and, hopefully, work out what was wrong with what they were doing.
CATHERINE: While we didn't realize the symbols were star constellations, it was obvious to us that they match symbols written on the star gate. Problem was, we never knew about the seventh symbol
Stargate: Draft Script
When it comes to it, the gate developers need to add a custom space for the extra symbol, something that they didn't realise they needed because they didn't realise that the symbols represented anything more than a (6 digit) code to be entered.
Gate Engineer: Programming seventh symbol into the computer
In the TV series, we learn that the dialling program is actually far more sophisticated than a device that simply turns the wheel and gives the gate power. There are a very considerable number of inputs and outputs, some of which they understand and some of which are baffling. I think we can assume that Col. West thought that Jackson would offer some insight into why their program was failing, not that the dialling symbols themselves were incorrect.
That all being said, the writers of the show were happy to acknowledge that this wasn't especially clear and wasn't a particularly well written aspect of the film.
Stargate SG1: 200.