How are you getting it on the slant?
Yeast is transferred to a slant by streaking cells taken from another pure culture source onto the surface of the slant using a nichrome loop. Contrary to what White Labs and Wyeast claim, their cultures are not 100% pure (no mass-produced culture is 100% pure 100% of the time), which means that one should plate liquid cultures for single colonies. Plating for singles is a standard laboratory procedure when transferring a liquid culture to slant. The cells are spread out on the plate using a one of several different streaking patterns.
Here's a video that demonstrates a three-sector streak: https://www.youtube.com/watch?v=Ay2hhujTuvg
The technique shown in the video is used to isolate yeast in addition to bacteria. The technique was originally developed to isolate bacteria, and was later adopted for isolating yeast.
Here's a plate that I streaked with a culture from Scottish and Newcastle's Tyneside Brewery:
The well-isolated round colonies in the lower right-hand corner of the photo shown above are all the offspring of single yeast cells; therefore, each colony is a single-strain pure culture. That's what we want to transfer to a blank slant using aseptic technique. Yeast will cover the entire surface (or at least most of it) of the slant after it has been streaked and incubated. From this point forward, we should not have to re-plate the culture. We only have to perform periodic sub-culturing to keep it alive. Sub-culturing is process by which yeast is taken from one slant and used to inoculate (streak) another slant using aseptic technique.
My goal is to be able to piggyback a starter propagation event on top of a sub-culturing event, which means that I attempt to use my cultures in a steady rotation, so that I do not have to perform sub-culturing-only events. Each culture needs to be sub-cultured to a new slant at least once every year. The more that I can piggyback making a starter on top of a slant-to-slant sub-culture the better. How frequently one brews will determine the number of cultures that one can maintain through normal use without resorting to pure sub-culturing events. I had over forty cultures in my first bank. I was sub-culturing all of the time. I am attempting to limit this bank to a dozen cultures. I usually keep two copies (two slants of each strain). I currently prepare twenty slants and six plates every time I make solid media, which means that I need at least 44 cultures tubes (I have multiples of this number at my disposal).