As @Lynn has mentioned, there as several threads on this topic that you can read to see other people’s experience.
Regarding your expectations of improving your application based on feedback, it seems you are assuming that there’s something wrong with your application or that the reason you have not been chosen has something to do with your application but you are competing with other applicants so think the owners may have just found a good match and looked no further. It could be that they are choosing sitters they already know (nobody would stand a chance there), it could be sitters are local (more peace of mind regarding possible delays for example), it could be age or gender (some owners do specify their preference, others don’t but that doesn’t mean they don’t have a preference. I know I have been chosen a couple of times because of my age. It is easier for them to believe I won’t be partying around and leave the pets unattended). These circumstances are beyond your control, so don’t take it personally. I am more often declined with no message at all, when I do get a message they usually just say thanks, they have chosen somebody else, a couple of times they said they had chosen someone local and once they said they preferred someone who spoke German (I don’t and this was in Germany)
Something we can control is what we choose to say in the application and our profile and that may make a difference (depending on the level of competition) In my opinion, an honest, personalized application that addresses all requirements of the listing and that focuses more on the pets and how we may be a good match is more likely to be successful.
This topic has also been discussed on the forum. There are different opinions regarding length and level of personal information but surely it will help you improve your applications.
Don’t get discouraged, we all get a fair amount of refusals with no message and still get offers for great sittings.
Good luck.