Clearly you're doing something wrong. But we've got no way of knowing what, since we can't see what you're doing. It could be just a matter of needing more practice. It took me a long time to get the one handed shuffle down due to having to work around small hands (it was literally quite difficult for me to simply hold the two packets of the deck in one hand for a long time). For that, it's probably a matter of how you're holding the deck and lining up the cards. One thing I've found that often helps is flipping the deck face up. The way many cards are cut, that makes the interweaving much easier, particularly if the deck is newer.