New inner tubes get too many flats
Two weeks ago I replaced my Kenda inner tubes to Scuwalbe #6 and in less than 100miles, I already got three flats, two in the front wheel and one in the rear. The first two flats were with the original Marathon tires and the third with new Kojack tires that I installed last week.
Some of the flats were in the outer side of the tube and some on the inner side. The interesting thing is that in all three cases I could not find any sign of puncture or other damage on the tires and the rim tape seems to be ok and with no sharp edges from the spoke holes.
The bike is a few months old BF NWT with Alex 20" rims and I never inflate to more than 90psi.
The only possible causes I can think of bad batch of Schwalbe inner tubes or that I damaged the new tubes somehow when installing them (I was very careful not to damage them and used plastic levers but who knows).
Any idea what can be the problem or what I should look for?