Nailed down floorboards in bathroom to stop them squeaking, but they still squeak what will i try now?

depends on whether its the boards rubbing on each other as they flex ( should not be a problem if they are tongue and groove as they should flex together (if at all) as you walk on them) or possibly the boards are a bit curved/warped and the nails are not holding them fast ( in which case screws might be better but drill the boards so the screws dont split the planks) , or is it possible that the floorboard is sitting on / rubbing on pipes directly beneath the floorboards and this is the noise you are hearing ?

Good luck - keep us posted.

squeaks really hard to solve without taking the floor up... in a bathroom the moisture can easily warp the boards so they lift a little. as you walk on them they compress back down and hence the squeak. one little word of caution 'Nailed down floorboards in bathroom' says nail through pipe to me... so be careful with what you send crashing through those boards or you might be asking us about repairing water damaged ceilings next...

Did you use "cut" nails as you should,

thanks for your help guys, i should have explained properly, i did drill into the floorboards and screwed them down next to where the original nail was and done a few more. the floorboards are large and i dont think i will be able to lift them to see under if they are sitting on a pipe, the floorboards either go under the bath, skirting or toilet. They are not tongue and groove just big flat ones! What are "cut" nails?

Those thin flat sided ones,wern't they called floor brads?

Cut nails are exactly that, cut out of flatbar they are tapered the whole length so that when used they wedge in the wood they have a flat "point" so as not to split the wood, whilst on the one hand they hold the board very well. on the other hand the shape
facilitates relatively easy uplifting of the board, Round or oval nails do not work as well on floor boards,
Screws work but take longer to fix and are more expensive,

When I ran an emergency responce firm
Satuday mornings inevitably brought with it nail through pipe jobs,

When you find a lose squeaky board look to see any evidence of it having been taken up, if it has
then there are probably cables or tube under it, you can then either use a detector to locate the exact
position of the pipe/cable or take it up and have a proper look. if the job has been done to spec then
any cables will run through holes bored through the center of the joist out of reach of nails,