Right now they're trying to get the best shield good enough, that and make engines that keep working and don't explode
Also they are making larger changes each flight, intruding new problems as they fix others
The upcoming flight test is expected to perform a stage 2 engine relight to prove it can deorbit itself, it wasn't able to prove that last test flight as stage 1 didn't deliver enough energy so stage 2 spent too much propellant and didn't have enough spare for the relight