The Gaps between the plate and the tips may be caused by bad tips, good tips with bad surface preparation, bad pretinning or any combination of the three. Carbide is made to be very resistant to wear and corrosion. This means that it resists chemical and physical attack very well. Pretinning and brazing are based on chemical and physical joining. You can braze some carbide without special preparation but it does not necessarily braze very well.
Good carbide needs special treatment to ensure excellent, consistent brazing and bonding.