HT
exactly this has happened to me in busy uk airspace. Freq change, check in, told to stand by, called again after a few minutes and told roger. Called again near FIR boundary and out of range. 121.5 resolved the issue and received an apology from senior controller over the phone (after she had heard the tapes).
Sometimes when its busy (and especially in some countries not far away) controllers do not listen/acknowledge properly.