The IS250 manual says that if you have problems with unlocking the car you can place the fob next to the little black lock button, the same with the ignition, place fob next to the start button. Maybe your key needs a new battery/batteries. I have not tried dismantling the fob, but perhaps you could check the battery voltage with a volt meter?
I had a problem with my last is200 se, in that whenever I used the keyfob to open the boot, it rang my next door neighbour's doorbell. I thought it quite funny, my neighbour didn't think so. Eventually fixed by selecting a different frequency for the neighbour's doorbell system.