Not necessarily the right forum, but there is a lot of experienced folks on it. There are DIY home forums too.
Pictures always help.
How big is the gap?
What type of silicon are you using? Link?
How much flex is there? I wouldn't expect anything to stick if there is major movement. You may need to glue the surround back into place to make it more stationary.
Often if it is a larger gap you need to put a backer rod in, which is a grey foam you can buy at big box stores.
If the silicon is releasing that fast there are probably contaminants on tub area. Take it all back off, use a scuffy pad to remove the remainder, turn the fan on at the least, but a respirator would be better, put neoprene gloves on, then get out the acetone and do a quick wipe of the area. Be careful the chemicals do not melt the surround. Cleanliness is probably the biggest factor here.
There also comes a point you let the landlord deal with it...