The "Classic" effects do not work on sounds that are played back at a sample rate lower than 44,100 Hz.
If you are rendering to a very low quality MP3 file and suddenly do not hear the effects, try rendering to a .WAV at 44,100 Hz first.
Then load in the .WAV and render to the low quality MP3.
If you have your mixer playback settings at a sample rate less than 44,100 Hz, please raise it to 44,100 Hz.. This would be in the Mixcraft Preferences.
An easy option, assuming your recording has a sample rate of 44100, is to freeze the track and then mix it down to the lower sample rate.