Average speed on coaster decreases?

I am on the penultimate level in career mode and trying to build a coaster with an average speed of at least 25mph with a monthly profit of at least £3000. I built it as per the specs and know it was fine when I opened it because the monthly profit of that ride was updated each month in the objectives. After a few of months I noticed that it no longer satisfied the criteria and on investigation realised it was because the average speed had dropped from 28mph to 23mph. I tried refurbishing the ride but it then dropped to 17mph!

This is the first time this has happened- does this mean it is a bug?? Everything else works fine. If not, is it something to do with the age/reliability of the ride? If so how can I fix it?

Any help greatly appreciated!!
 
Do you have any block-brakes or brakes in general on the course? Do you have multiple trains that come to a stand-still (maybe in front of the station)?
What kind of coaster is it?
 
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