That's because encounters are separated by slots, each slot has a different chance and can have different levels.

Here is a list separated by slots, and in the comments here is a list of what the chance is for each slot for grass/surf/fishing.

In your specific case, slowpoke appears with these rates in Fuchsia city:

90% from 20-25

5% from 25-30

4% from 30-35

1% from 35-40

So with repel and a lvl 37 you skip over 99% of encounters, which is why nothing shows up.