Slack string on linear slides when using a different slide lengths

We have multiple 400mm length linear slides as used in the FIRST tech challenge robotics. However, our very last and innermost set of slides are 300mm length mounted such that they are not closer to the end of the extension (the black slides in the picture). We are having a lot of trouble keeping the string tensioned regardless of if we use springs (not currently in the photo). We were wondering if the two different extension length slides we used would contribute to this but we’re not sure why that would be. I would love to elaborate on this further but I’m not sure what details would be important.

Quietrabbit said:

We have multiple 400mm length linear slides as used in the FIRST tech challenge robotics.

Please provide a web link to the "FIRST tech challenge robotics", so we can see how the slides are arranged.

Quietrabbit said:

... We were wondering if the two different extension length slides we used would contribute to this but we’re not sure why that would be. I would love to elaborate on this further but I’m not sure what details would be important.

We neither.
Please, show us the whole picture in more detail.