Have you confirmed that these reports are defined exactly the same (e.g., timeout value, maximum number of rows, etc)? If yes, could it be due to the poor performance of your 7.1.9 environment? Normally comparing alert logs should tell you why certain reports time out. You should see error stack in the pega logs as well.

There could a number of reasons why a report is timing out. The indexes might need a rebuild, the statistics could be stale, too much fragmentation of the table etc. As Kevin suggests, the alert log is a good start. If you are using AES or PDC, it could even show if indexes are present or not. Doing an explain plan could also help find the problem for you.