How to lookup values between a delimiter and replace based on range?

I apologize for not explaining better. I hope this makes more sense.

I thought that the values were not being written in Column C if an ID from Column B was referenced and didn't exist.

What I didn't realize until I physically clicked in one of the cells is that it's writing the values, but for some reason the values in the cell are not visible unless I click on it; then, the value is only visible in the formula bar. I've tried changing the formatting of the affected cells and I still can't see them, however if I copy the cell to another worksheet, it becomes visible. Strange, but it can be overcome by copy/paste.

What I can't seem to overcome is if one of the id's doesn't exist, the function isn't inserting a comma between the values that do exist. I've uploaded a range of cells that I copied from the master sheet. On row 33 you can see that Part Numbers 8703 & 8704 were copied, but no comma was inserted between them.

Looking at the code I can see that you're splitting the values between the semi-colons, doing the lookup, then on line 9 inserting a comma between the returned values. For some reason this isn't working when one of the id's doesn't exist. I'm not sure where to begin to fix that.