How did you set the default Plymouth splash ? What command did you type ?
Did you update the initramfs for the current kernel, or all installed kernels, and are you loading the correct initramfs at boot ?
Do you have an Nvidia graphics card, if so are you using the proprietary driver or the open source Nouveau driver ?
What boot options do you have in your grub configuration file, particularly for gfxmode, gfxpayload ?