Perhaps try this approach?
Take a single note eg D on the G string. Get your Spiccato going smoothly and cleanly, examining the bow and string movements by looking down and listening. Try different contact points, heights of bounce, lengths of strokes and angles of descent (attack). Also try different parts of the bow hair, usually in the "comfort zone" (no more than 1/2 way out and no closer than about 4" from the frog). Each bow is different WRT its "sweet spot". Speed will have a big impact on the above variables. Work up to performance speed in steps, perhaps practicing with a metronome.
Next repeat the process using two notes on the same string. Then two notes across two strings, as in Zimmerman's book. If you can't do these simple exercises cleanly and confidently at performance speeds then you may have less success when you add in the actual passages that involve more complexity of fingering, changing position and bowing.
At the same time practice these passages with on-the-string and semi-off-the string bowings. These will give you more powerful dynamics in the lower passages on the middle two strings. Shorten bow lengths until you feel the bow hair tug (bite) at the beginning of each direction and yet you are still producing a musical sound. Too much bow and you skid, too little and you crunch. Shortening your bow lengths also helps to balance the work being done by each hand and make timing and coordination cleaner (therefore articulation clearer).
Bow tension might be another factor, also bow quality and style. Are you German or French bow user?
Bow closer to the end of the fingerboard on the E string. Here it is more flexible and easier to move for faster notes in any style. Conversely, stay away from the end of the fingerboard on the G string. Remember that the strings are springs, the bow stick and the bow hair are also springs, so allow your bow hold to also be sensitive, flexible and spring-like.
My final observation is that these passages are very popular audition pieces that require you to play strongly in style as well as powerfully through-out with all notes sounding clearly. True spiccato may not give you these results and you might be better off with semi-off bowing that allows more arm weight?
Good hunting!!
DP