in an experiment, the compressive strength of the concrete reduced on day 14, compared to the strength obtained on day 7. why is this so?

I think you have not done curing of concrete continually for 28days!

Have you stopped curing after 7days due casting?

If so then the concrete will not gain its strength, as we know that the cement is very greedy for water to hydrate, but because you stopped curing after 7days then it was not able to get the moisture and the concrete got shrinkage cracks and also due to creep the strength has been reduced!

So its very important to cure the concrete for 28days to gain the strength!