What kind of rim tape do you have? Some rim tape like Velox makes can make it extremely difficult to mount some type of tires. I had these DT Swiss rims and it was impossible to mount any kind of tire and would take me 20 to 30 mins of struggling, sweating, etc to mount a tire. I finally switched to using a thinner rim tape by Zipp and then I could mount tires way easier. If you are using Velox, try looking for some high pressure rim tape. Now a days I use Veloplugs if the rim holes are the right size. Also makes mounting tires so much easier.