Many baths aren't properly supported, which means that when full, they move away from the wall and break the sealant. When this happens, you can call a plumber -- the expensive choice -- or you can replace the sealant around your bath yourself. With just a few tools and products, you can properly anchor your bath and replace the sealant. Once you have done this correctly, you can rest assured that your bath will stay anchored to the floor and stuck to the adjoining wall.
Remove all the old silicone completely; don't leave any trace of it. You can remove it either by slicing it away from the edges of the bath with a sharp knife or you can use a product like silicone eater that is specially designed to remove all silicone or sealant products quickly.
Ensure the surfaces you will be putting the sealant on or beading are completely clean and dry. You can do this by wiping it with white spirit.
Ensure the bath is properly supported. To do this you will need to remove the side panel and look at the bath. Some of them are only standing on their legs with perhaps a thin metal frame around the midsection of the bath. This causes the bath to move away from the wall when it is full due to the weight, therefore breaking the sealant. To avoid this, put some bricks in between the bottom of the bath and the floor. Make sure the bricks fit snugly between the bath and the floorboards to reduce movement when the bath is full.
Replace the side panel of the bath.
Fill the bath until the water reaches just below the overflow level. This will pull the bath slightly away from the wall.
Apply a constant bead of silicone around the edges of the bath that are to be sealed to the wall. Do this by placing the silicone tube into the sealant gun. Place the nozzle in one corner of the bath and the wall and slowly squeeze the trigger laying the sealant in small beads between the bath and the wall. Slowly move the gun while squeezing until you have covered all the areas you want to join. Let the silicon dry for between 10 and 15 minutes.
Cover your forefinger in liquid detergent and run that finger along the bead of silicone. Keep your finger covered with the dish-washing liquid until you have finished covering the sealant so that you end up with a smooth, neat join.
Leave the silicone to dry for a day or two then wipe it clean with a damp cloth.
Be sure not to apply too much sealant, which will make a mess.