Depending on the model there will be a few troubleshooting steps we could try.
For the classic series you really want to make sure that your rollers are fully expanded.
If it is an Econ - there is a latch behind the pump head we need to make sure is fully secured.
For anyone interested, I have...
We do not suggest using a 'Y' as there is a chance of leaking at the connection. In regards to using the 3/8" tubing, the 1/4" is fine as the pump is a positive displacement pump and increasing the tubing size does not affect the output, unless a viscous solution. I hope this might help.