Increase range of 433MHz remote sensor
"I didn't provide much information about the circuits because I'm not sure if it matters in my case"

Oh yes that matters ! There are several 433MHz receiver modules you can buy on ebay etc. I have a simple one:



And a more complex one:



Mine is similar shape but with a metal shield covering the components.
I have never seen the simple model (first picture) with a shield.

If you have the simple module, then that is the reason why you experience poor range. The more complex module has a better receiver with AGC (Automatic Gain Control) this helps a lot.

For maximum range you want to use an antenna of the proper length of 1/4 * lambda, at 433 MHz this comes to 17 centimeters. It is best to stretch out the antenna to its maximum length but it will also work when coiled up a bit.