What do you mean by "it sounds bad"? Can you be more specific? Does it sound good when you slap on other basses? If no, then practice slapping
, and if yes, maybe use your bass-that-sounds-good-for-slap for slap, and use the P bass for finger/pick style?
I recommend you always put on a fresh set of strings and do a full setup whenever you buy a new-to-you instrument. Definitely do that first before you judge the bass.
Be patient. It will take time for you to adjust to the new bass. What type of bass did you switch from?