Individual armored infantry and exotroop units weigh more than the 20 ton capacity of a jumptransport, so they don't fit one to a transport. When a fleet transporting these undergoes attrition, the mass of all transported infantry is usually slightly larger than the capacity of the transports carrying them. This makes it impossible to deploy any ships, even non-transports, out to a new fleet; one of the two resulting fleets will be overweight and the deployment will not be allowed to go through.

Even if there are enough transports in the combined fleet, it might not be possible to divide the transports since there is no way to for all the cargo to fit into two fleets because one resulting fleet would contain slightly too much. (You would be able to deploy other ships in such a scenario, just not transports.)

The workaround is to always send more transports than are needed, but this is not intuitive.

I would suggest making the masses of all transportable resources be factors of 20, and that some sort of mechanism be put in place to prevent transport fleets from going overweight (forced attrition of cargo?) or to allow overweight fleets to split.