Symptop:
When copying an opportunity or creating a new opportunity from an old opportunity, it is observed that the sales cycle, forecast or classification data is not copied to the new opportunity.

Cause:
SAP GUI -> SPRO -> Customer Relationship Management -> Transactions -> Basic Settings -> Copying Control for Business Transactions-> Define Copying Control for Transaction Types
Copy control from opportunity transaction type to opportunity transaction type is not maintained properly.
In copy control from opportunity to opportunity, there are three flags, "Copy sales cycle", "Copy forecast", and "Copy classification". You need to check the relative flag to copy relative part. For example, you need to check "Copy forecast" so as to copy "Forecast" data.

Solution:
Scenario 1 -> copy an opportunity
Assume the opportunity transaction type is ZOPT. "ZOPT to ZOPT" needs to be maitnain in copy-control. "Copy sales cycle", "Copy forecast", and "Copy classification" need to be checked so as to copy sales cycle, forecast or classification data.
Scenario 2 -> follow-up an opportunity
Copy-control must have been maitnained. You need to find the setting and check "Copy sales cycle", "Copy forecast", and "Copy classification".
You may refer to SAP Note 1901588 - Copying opportunity does not copy data in "sales cycle", "forecast", or "classification".