You could put on a brand new set of Spirocore Weichs. Spend a week or so on those, then put your old ones back on. Suddenly, the thud will sound nice.
But seriously, like isolated said. It's probably your strings - and/or set up issues. What do you have on? Are they the strings that were on when you got the bass? Do you have a teacher that can help you? A luthier? Do you know someone who has been playing a long time that can help?
Might be best to get someone to walk you through string choices, how to change them, and a look at your bass to make sure it is healthy.