well...that is extremely strange. wheelspin at the front but not rear is pretty much unheard of, unlike many 4wd systems, attessa cannot put more than 50% to the front.
so unless you have some crazy wheel alignment (you already said you now have the same tyres all round), I think it is most likely to be a diff problem. It is possible you have a different ratio diff front to rear, and that might cause the issue you are talking about.
best to get a good workshop to check it out. if you want to check diff ratios yourself, get the car off the ground, and have 1 person turn the rear wheels while 1 person watches the tailshaft turn. You should get 4.1 turns of the tailshaft for 1 turn of the wheels if it is a standard diff. The most likely other options are 4.4:1 (gts4), 4.3:1 (stagea) of 3.8:1 (r34 gtr, I think)? Then try the same test on the front wheels. Different ratios front and rear will be a disaster for attessa, it is designed around rear wheel spin and moving to the front, never front wheel spin.
BTW that is all assuming you don't have a 1000hp drag car. If you do....you will have to work it out for yourself