If you've gone finer, added more coffee, and used boiling hot water, but it's still weak, check a few things:

• Is your coffee fresh? Stale beans lose flavor fast, and heat speeds that up.
• Is your grind actually consistent? A cheap grinder might produce a mix of fine and coarse pieces, which leads to uneven extraction.
• Are you pressing too soon? Give it the full four minutes. Patience matters.

If it's still watery after all that, try a different bean. Some origins are naturally lighter-bodied, and that's not a fixable problem—it's just a different cup.