The range printed on a monitor box is almost always tested outdoors with a clear line of sight — real houses have walls, floors, and other electronics, all of which cut that number down significantly. A "300m range" monitor might struggle at 15 metres through two brick walls and a floor.
Common interference sources: Wi-Fi routers, microwaves, cordless phones, and other baby monitors on the same frequency in a nearby flat or house. If your monitor cuts out at a consistent time of day, check what else switches on around then.
If range is a genuine issue in your home, a monitor using DECT technology (the same as most modern cordless phones) tends to handle walls and interference better than basic analogue models, worth checking the spec before buying.