Everyone's anatomy is basically the same, but they all come in different shapes and sizes.. my guess is that you're big (you said you needed maxim condoms) and maybe she is really tight and not very deep. So entering her may just be difficult because of your different shapes.. could be a possibility..
Also, when you're trying to enter her are you completely hard? If you aren't, it is nearly impossible to go inside... make sure your erections are very firm before you start..
Hope this helps