Hello again - When I was doing shoe repair we eould approach this one of two ways. One method was to "strip" the old polish (and sometimes some of the balck) with laquer thinner, then re-dye the entire boot, then apply the polish and buff. Another option is to use a hair dryer or a heat gun to heat the polish. When heated you can choose to wipe the excess off and re-polish or sometimes the heat will allow the polish to soften enough to eliminate the"cracking". Most polishes have a high concentration of waxes in them which is likely what is causing the "cracking"

If the leather itself is cracking under the polish you may need to apply a conditioner to the leather (after cleaning) to get the leather to return to its original smooth finish. Then apply the polish. Sometimes when the leather itself is cracked badly you cannot "bring them back" and sadly you need to replace the boots.

I would start with heating the polish and if more drastic measures are needed proceed from there with stripping or the other methods mentioned above.

Let me know how it goes or if you need more help on this