Different makes and models of TV sets have different 'sensitivities'. (i.e. their ability to handle weak signals). The TV in your bedroom would seem to have a better selectivity.
It's also possible that the longer lead, going to the kitchen, might be relevant. There are inevitable signal losses along any length of coaxial cable. Further, signal break-up can also be due to a TV set struggling to separate the weak signal that it wants from electrical interference in the area. Your fridge, freezer or microwave oven (among other appliances) might be adding to the problem in the kitchen.
Ideally you need to upgrade the aerial but, if the existing splitter in the aerial cable doesn't incorporate an amplifier, it might be worth investing in one. (They're under £20).
Chris
PS: The signal strength at your aerial doesn't remain constant. It's affected by the 11-year sunspot cycle, individual solar flares, high atmospheric pressure, leaves on deciduous trees between the transmitter and your aerial, local electrical interference and much else. So you might get a decent signal one day but, if your aerial isn't up to the job, an inadequate one the next day.