What exactly is happening? I assume you are using MSP 2013 and not SP1, because SP1 is not supported at 13.3?
Anyway, depending on how you are doing your forecast in MSP you may be running into one of several defects we have encountered, but without more detail, it is hard to tell.
How do you do your forecast in MSP?
how are you then creating cost plans form that forecast in Clarity?
When you reopen your project in MSP, is the forecast correct? ( again, depending on what you are doing, defects exist around this)
Are you using Fixed duration, units or work?
ETc
the more information you can provide, we might be able to help more.