AnalogIndustriesBlog said: "
Tracktion, specifically, is a bit odd, in our experience. The thing is that it heavily uses (or "used" might be more specific) an oft-ignored "feature" of VST. There are two methods of performing the actual DSP of a plug-in in VST land. They are called process() and processReplacing(). Before VST2.4, when process() was deprecated, only the oldest hosts used that method. Up until VST2.4 was released, you had to have both methods. They were essentially identical, except in the final sum you'd do output = dspStuff in process() and output += dspStuff in processReplacing().

I could provide further explanation, but if you're not a programmer, it would be uninteresting, and if you are a programmer, you already know it. Suffice to say, Tracktion uses (or used) process() in order to provide the "feature" (we call it a bug) of having a wet/dry mix for every effect. When the rest of the VST world Switched to VST2.4, Tracktion was a little slow to catch up, and it was looking for process() where there was none.

Interestingly, this is also the reason that you can't use newer effects as a send in any Cubase of SX1 or prior. The work-around is to make a group, and insert the effect in that group, and send to the group.
"