The triac (BT1308W) on OSPi can only supply a maximum of 800mA continuous current per channel. In practice this may be lower due to factors such as the base current. I am curious what type of solenoid valve you have — typical sprinkler valves have an operating current of 150mA to 250mA. I’ve never seen a valve that takes as much as 1A current.
There are two work-arounds:
1) you can combine several zones in parallel to increase the current limit. For example, combine zones 1 to 3 together to triple the current output. The downside is that you need to set zones 1 to 3 to always turn on at the same time, and also this wastes 2 zones.
2) alternatively you can replace the BT1308W by a more powerful triac, such as BT137 / BT138. Keep in mind that if you ever run more than 2A current, it will trigger the on-board fuse, so you may have to replace it with something bigger, or simply short the fuse to bypass it.