Why wont my transmission shift properly?

My car wont shift on time unless I let off the gas. It will shift eventually but not until like 4.5k RPMs. Its mostly when im merging onto the highway, I think from 3rd into 4th. I do have a slight leak somewhere but the fluid level is fine. Ive done a "transmission fluid exchange" and ive added a Lucas treatment with no results. Should I replace the transmission filter or is it something else? Thanks