I am not a plumber
The house we rent has three toilets. Two had their flush fail, we should have had the third fixed at the same time. Now the third tends to fail too and the present lockdown makes me disinclined to have our landlord seek a plumber for anything other than a true emergency.
On closer inspection that third toilet seemed simply not to be refilling quite enough any more. The float is on a kind of plastic screw so I raised it to a higher level and that does seem to have helped. However, there is some other issue where, even with plenty of water, sometimes the flush can't get started fully. On the intake pipe I have found what looks to be a valve that allows me to cut off water to only that cistern but I shan't try to replace the mechanism myself, the chance is too high that I will simply end up with a useless toilet.
On closer inspection that third toilet seemed simply not to be refilling quite enough any more. The float is on a kind of plastic screw so I raised it to a higher level and that does seem to have helped. However, there is some other issue where, even with plenty of water, sometimes the flush can't get started fully. On the intake pipe I have found what looks to be a valve that allows me to cut off water to only that cistern but I shan't try to replace the mechanism myself, the chance is too high that I will simply end up with a useless toilet.