Both Yugoslavia and Romania had very important economic relations with Germany, which bought a lot of their products. Their interest was to avoid a direct confrontation with Germany and to remain neutral.Andy H wrote: It would certainly be interesting if they were to honour such a pact. Would not Germany's influence/power make them both very wary of siding with Czechoslovakia given Hungarian involvement?
King Carol II noted in his diary on 23 May 1938 that as long as Hungary does not get involved in a war with Czechoslovakia, he could continue his politic to remain neutral and not engage in a war with Germany. The Romanian Minister at Berlin, Radu Djuvara, was instructed to communicate to the German government that the Romanian government would be put in a difficult position if Germany would use Hungarian soil to attack or would instigate Hungary to attack Czechoslovakia. However, it seems that the king had decided on 26 December 1938 not to intervene (this information was taken from a secondary source, which I do not have). Still, my opinion is that Carol could have been influenced to intervene in case France and England went to war and Hungary attacked Czechoslovakia. This is why the German Foreign Affairs Minister ordered that the Hungarian government be advised not to engage in any actions that would make Yugoslavia and Romania to go to war.
There was also the problem of Soviet intervention and the right to pass through the country. The Romanian government feared that the Red Army once on Romanian soil, would eventually decide to remain in Bessarabia.