It is not uncommon for a pet to decide they do not want to eat for a day or so. We tend to attribute it to their sudden ""dislike"" for a food, when, in reality, it could be for host of different reasons that they are unable to communicate to us. The biggest mistake we as pet parent's make is to panic thinking that they will go hungry and we try introducing some new food to them as an alternative. Believe it or not, this may be why they are not eating because they have learned the cause and effect of this behavioral manipulation.
Q&A:
Q: I try to heat up leftover cat food a bit in the microwave but my cat can tell it's not the same as a fresh new can and wont eat it.
A: Here are some suggestions that may help with getting your cat to eat the leftovers.
If you're not already using lid covers on the refrigerated food, lids could help. The big reason that microwaving works (in most cases) is that it helps to unleash the aroma that is naturally contained in the food. These lids will keep oxygen out and those aromatic compounds sealed in the can, which helps to maintain the freshness.
When a portion of wet food is stored in the fridge, the refrigeration process may also be removing a little moisture if a lid cover is not used.
If you're already using a lid, you could try adding a little chicken broth to the food and then microwave it for a bit. If chicken broth is not available, warm water could help as well. The key difference is that water is flavorless, so adding water to restore moisture may ultimately dilute the flavor for your cat whereas chicken broth adds a little flavor back into the can while bringing moisture as well.
Here is an example of some covers at Amazon.com: https://www.amazon.com/SHENQIDZ-Covers-Universal-Dishwasher-Silicone/dp/B095M6H5D7/ref=sr12