May be you are using high melting point fat or using low temperature in nozzle area, for this reason cream viscosity increasing.
High viscosity of fluid means lower the flow rate which create blocking.
May be increasing the emulsifying agent help you.
Assuming that your cream does not have hard particles (like from sugar) it could be that slowly fat is crystallising in the nozzle. This can be caused either by too low temperature of the cream or by too cold nozzle.
To build on Gabrie's answer - you might also want to consider
- the solid fat profile of the fat and the crystallisation dynamics (has it changed, is it fit for purpose?)
- if you are experiencing any pre-crystallisation of the cream before the deposit
- how slight changes to the formulation may make the cream more prone to solidification (has the level or type of fat changed? Have levels of non fat ingredients changed?)
It would be useful to know if there have been any changes to your process recently that have coincided with your observation - are you running for longer? Are you running at a different time of the day/week/year?
To add on the good answers above, are you using any rework in the cream? If yes can you pls detail.